diff --git a/compiler/lowering.go b/compiler/lowering.go
index 7fc30e21a..4aeee6686 100644
--- a/compiler/lowering.go
+++ b/compiler/lowering.go
@@ -6469,8 +6469,11 @@ func lowerCompoundAssignValue(
 	tok token.Token,
 	mask bool,
 ) string {
-	// Wide integers, int bitwise operations, 32-bit products and integer
-	// quotients have helpers.
+	// Complex numbers, wide integers, int bitwise operations, 32-bit
+	// products and integer quotients have helpers.
+	if helper, ok := complexArithmeticHelper(targetType, tok); ok {
+		return complexHelperCall(runtimeOwner, targetType, helper, left, right)
+	}
 	if helper, ok := wideIntegerAssignHelper(targetType, tok); ok {
 		return wideIntegerHelperCall(runtimeOwner, targetType, helper, left, right)
 	}
@@ -9874,6 +9877,46 @@ func (o *LoweringOwner) lowerComplexEqualityExpr(ctx lowerFileContext, expr *ast
 	return value, true
 }
 
+// lowerComplexArithmeticExpr lowers an arithmetic operation of complex
+// numbers, which are objects of their parts, to its runtime helper.
+func (o *LoweringOwner) lowerComplexArithmeticExpr(ctx lowerFileContext, expr *ast.BinaryExpr, left string, right string) (string, bool) {
+	resultType := ctx.semPkg.source.TypesInfo.TypeOf(expr)
+	helper, ok := complexArithmeticHelper(resultType, expr.Op)
+	if !ok {
+		return "", false
+	}
+	return complexHelperCall(o.runtimeOwner, resultType, helper, left, right), true
+}
+
+// complexArithmeticHelper returns the runtime helper of the arithmetic
+// operator op, or its assignment form, on complex numbers of type typ.
+func complexArithmeticHelper(typ types.Type, op token.Token) (RuntimeHelper, bool) {
+	if !isComplexType(typ) {
+		return "", false
+	}
+	switch op {
+	case token.ADD, token.ADD_ASSIGN:
+		return RuntimeHelperComplexAdd, true
+	case token.SUB, token.SUB_ASSIGN:
+		return RuntimeHelperComplexSub, true
+	case token.MUL, token.MUL_ASSIGN:
+		return RuntimeHelperComplexMul, true
+	case token.QUO, token.QUO_ASSIGN:
+		return RuntimeHelperComplexDiv, true
+	}
+	return "", false
+}
+
+// complexHelperCall calls the complex helper with the operands, rounding a
+// complex64 result to the precision of its float32 parts.
+func complexHelperCall(runtimeOwner *RuntimeContractOwner, typ types.Type, helper RuntimeHelper, operands ...string) string {
+	call := runtimeOwner.QualifiedHelper(helper) + "(" + strings.Join(operands, ", ") + ")"
+	if basic, ok := types.Unalias(typ).Underlying().(*types.Basic); ok && basic.Kind() == types.Complex64 {
+		return runtimeOwner.QualifiedHelper(RuntimeHelperComplex64) + "(" + call + ")"
+	}
+	return call
+}
+
 // lowerPointerEqualityExpr compares two pointers, also unsafe pointers.
 // Pointers to the same storage are the same object, except references to a
 // slice or array element, which each &s[i] makes anew, and views that an
@@ -10216,6 +10259,9 @@ func (o *LoweringOwner) lowerExpr(ctx lowerFileContext, expr ast.Expr) (string,
 		if _, ok := typed.Y.(*ast.BinaryExpr); ok {
 			right = "(" + right + ")"
 		}
+		if value, ok := o.lowerComplexArithmeticExpr(ctx, typed, left, right); ok {
+			return value, append(leftDiagnostics, rightDiagnostics...)
+		}
 		if typed.Op == token.AND_NOT {
 			// Wide (int64/uint64) operands lower to bigint, and a Go constant like
 			// 1<<63 lowers to a JS number whose raw ~ collapses to -1, so the raw
@@ -10302,6 +10348,11 @@ func (o *LoweringOwner) lowerExpr(ctx lowerFileContext, expr ast.Expr) (string,
 			return "await " + o.runtimeOwner.QualifiedHelper(RuntimeHelperChanRecv) + "(" + value + ")", diagnostics
 		}
 		value, diagnostics := o.lowerExpr(ctx, typed.X)
+		if typed.Op == token.SUB {
+			if resultType := ctx.semPkg.source.TypesInfo.TypeOf(typed); isComplexType(resultType) {
+				return complexHelperCall(o.runtimeOwner, resultType, RuntimeHelperComplexNeg, value), diagnostics
+			}
+		}
 		if typed.Op == token.NOT || typed.Op == token.SUB || typed.Op == token.ADD {
 			return lowerPrefixUnaryExpr(typed.Op, value), diagnostics
 		}
diff --git a/compiler/runtime-contract.go b/compiler/runtime-contract.go
index ca3e9ece0..dd46b594d 100644
--- a/compiler/runtime-contract.go
+++ b/compiler/runtime-contract.go
@@ -55,6 +55,11 @@ const (
 	RuntimeHelperMax            RuntimeHelper = "builtin.max"
 	RuntimeHelperComplex        RuntimeHelper = "builtin.complex"
 	RuntimeHelperComplex64      RuntimeHelper = "builtin.complex64"
+	RuntimeHelperComplexAdd     RuntimeHelper = "builtin.complexAdd"
+	RuntimeHelperComplexSub     RuntimeHelper = "builtin.complexSub"
+	RuntimeHelperComplexMul     RuntimeHelper = "builtin.complexMul"
+	RuntimeHelperComplexDiv     RuntimeHelper = "builtin.complexDiv"
+	RuntimeHelperComplexNeg     RuntimeHelper = "builtin.complexNeg"
 	RuntimeHelperReal           RuntimeHelper = "builtin.real"
 	RuntimeHelperImag           RuntimeHelper = "builtin.imag"
 	RuntimeHelperUint64Shl      RuntimeHelper = "builtin.uint64Shl"
@@ -393,6 +398,11 @@ func runtimeHelperContracts() []RuntimeHelperContract {
 		runtimeHelper(RuntimeHelperMax, "max", RuntimeHelperCategoryBuiltin),
 		runtimeHelper(RuntimeHelperComplex, "complex", RuntimeHelperCategoryBuiltin),
 		runtimeHelper(RuntimeHelperComplex64, "complex64", RuntimeHelperCategoryBuiltin),
+		runtimeHelper(RuntimeHelperComplexAdd, "complexAdd", RuntimeHelperCategoryBuiltin),
+		runtimeHelper(RuntimeHelperComplexSub, "complexSub", RuntimeHelperCategoryBuiltin),
+		runtimeHelper(RuntimeHelperComplexMul, "complexMul", RuntimeHelperCategoryBuiltin),
+		runtimeHelper(RuntimeHelperComplexDiv, "complexDiv", RuntimeHelperCategoryBuiltin),
+		runtimeHelper(RuntimeHelperComplexNeg, "complexNeg", RuntimeHelperCategoryBuiltin),
 		runtimeHelper(RuntimeHelperReal, "real", RuntimeHelperCategoryBuiltin),
 		runtimeHelper(RuntimeHelperImag, "imag", RuntimeHelperCategoryBuiltin),
 		runtimeHelper(RuntimeHelperUint64Shl, "uint64Shl", RuntimeHelperCategoryBuiltin),
diff --git a/gs/builtin/builtin.ts b/gs/builtin/builtin.ts
index 228bfc72d..72db840db 100644
--- a/gs/builtin/builtin.ts
+++ b/gs/builtin/builtin.ts
@@ -628,6 +628,112 @@ export function complex64(
   return { real: float32(valueOrReal), imag: float32(imag) }
 }
 
+// complexOperand returns the complex number of an operand of a complex
+// operation, which is a number for a constant of an integer or float value.
+function complexOperand(value: Complex | number | bigint): Complex {
+  return typeof value === 'object' ? value : { real: Number(value), imag: 0 }
+}
+
+// complexAdd returns a + b.
+export function complexAdd(
+  a: Complex | number | bigint,
+  b: Complex | number | bigint,
+): Complex {
+  const x = complexOperand(a)
+  const y = complexOperand(b)
+  return { real: x.real + y.real, imag: x.imag + y.imag }
+}
+
+// complexSub returns a - b.
+export function complexSub(
+  a: Complex | number | bigint,
+  b: Complex | number | bigint,
+): Complex {
+  const x = complexOperand(a)
+  const y = complexOperand(b)
+  return { real: x.real - y.real, imag: x.imag - y.imag }
+}
+
+// complexMul returns a * b.
+export function complexMul(
+  a: Complex | number | bigint,
+  b: Complex | number | bigint,
+): Complex {
+  const x = complexOperand(a)
+  const y = complexOperand(b)
+  return {
+    real: x.real * y.real - x.imag * y.imag,
+    imag: x.real * y.imag + x.imag * y.real,
+  }
+}
+
+// complexDiv returns a / b as the Go runtime computes it (complex128div):
+// by Smith's algorithm, with the infinities and zeros of C99 Annex G when
+// both parts of the quotient are NaN. A division by zero does not panic.
+export function complexDiv(
+  a: Complex | number | bigint,
+  b: Complex | number | bigint,
+): Complex {
+  const n = complexOperand(a)
+  const m = complexOperand(b)
+  let e: number
+  let f: number
+  if (Math.abs(m.real) >= Math.abs(m.imag)) {
+    const ratio = m.imag / m.real
+    const denom = m.real + ratio * m.imag
+    e = (n.real + n.imag * ratio) / denom
+    f = (n.imag - n.real * ratio) / denom
+  } else {
+    const ratio = m.real / m.imag
+    const denom = m.imag + ratio * m.real
+    e = (n.real * ratio + n.imag) / denom
+    f = (n.imag * ratio - n.real) / denom
+  }
+  if (!Number.isNaN(e) || !Number.isNaN(f)) {
+    return { real: e, imag: f }
+  }
+
+  // Correct the result to infinities and zeros where they apply.
+  let [x, y, c, d] = [n.real, n.imag, m.real, m.imag]
+  const isInf = (v: number): boolean => v === Infinity || v === -Infinity
+  if (c === 0 && d === 0 && (!Number.isNaN(x) || !Number.isNaN(y))) {
+    e = copySign(Infinity, c) * x
+    f = copySign(Infinity, c) * y
+  } else if (
+    (isInf(x) || isInf(y)) &&
+    Number.isFinite(c) &&
+    Number.isFinite(d)
+  ) {
+    x = copySign(isInf(x) ? 1 : 0, x)
+    y = copySign(isInf(y) ? 1 : 0, y)
+    e = Infinity * (x * c + y * d)
+    f = Infinity * (y * c - x * d)
+  } else if (
+    (isInf(c) || isInf(d)) &&
+    Number.isFinite(x) &&
+    Number.isFinite(y)
+  ) {
+    c = copySign(isInf(c) ? 1 : 0, c)
+    d = copySign(isInf(d) ? 1 : 0, d)
+    e = 0 * (x * c + y * d)
+    f = 0 * (y * c - x * d)
+  }
+  return { real: e, imag: f }
+}
+
+// copySign returns a value with the magnitude of v and the sign of sign,
+// a negative zero counting as negative.
+function copySign(v: number, sign: number): number {
+  const negative = sign < 0 || Object.is(sign, -0)
+  return negative ? -Math.abs(v) : Math.abs(v)
+}
+
+// complexNeg returns -a.
+export function complexNeg(a: Complex | number | bigint): Complex {
+  const x = complexOperand(a)
+  return { real: -x.real, imag: -x.imag }
+}
+
 export function real(value: number | Complex | null | undefined): number {
   value = unwrapGoValue(value)
   if (typeof value === 'number') {
diff --git a/gs/fmt/fmt.ts b/gs/fmt/fmt.ts
index 5c0100a70..af0b0f367 100644
--- a/gs/fmt/fmt.ts
+++ b/gs/fmt/fmt.ts
@@ -309,19 +309,73 @@ function floatBits(value: unknown, typeInfo?: $.TypeInfo | string): number {
   return 0
 }
 
-// formatFloatVerb formats a float for the e, E, f, F, g and G verbs, with an
-// optional precision, as in Go.
+// formatFloatVerb formats a float or complex number for the e, E, f, F, g
+// and G verbs, with an optional precision, as in Go.
 function formatFloatVerb(
   value: unknown,
   verb: string,
   precision: string,
 ): string {
-  const bits = floatBits(value) || 64
   let prec = precision === '' ? -1 : parseInt(precision)
   if (prec < 0 && verb !== 'g' && verb !== 'G') {
     prec = 6
   }
-  return $.formatFloat(Number(value), verb === 'F' ? 'f' : verb, prec, bits)
+  const fmtVerb = verb === 'F' ? 'f' : verb
+  const cbits = complexBits(value)
+  if (cbits !== 0) {
+    return formatComplex(basicValue(value) as $.Complex, fmtVerb, prec, cbits)
+  }
+  const bits = floatBits(value) || 64
+  return $.formatFloat(Number(value), fmtVerb, prec, bits)
+}
+
+// complexBits returns 64 or 128 if value is a complex64 or complex128, as
+// the given type, the type it is boxed with or else its form tells, or 0
+// otherwise.
+function complexBits(value: unknown, typeInfo?: $.TypeInfo | string): number {
+  const info =
+    typeInfo ??
+    (value !== null && typeof value === 'object' ?
+      (value as { __goTypeInfo?: $.TypeInfo | string }).__goTypeInfo
+    : undefined)
+  const name =
+    typeof info === 'string' ? info
+    : info?.kind === $.TypeKind.Basic ? info.name
+    : undefined
+  if (name === 'complex64') return 64
+  if (name === 'complex128') return 128
+  if (name !== undefined) return 0
+  const kind = typeof info === 'object' ? info.kind : undefined
+  if (kind !== undefined && kind !== $.TypeKind.Interface) return 0
+
+  // A complex number is a plain object of its two parts.
+  const v = basicValue(value) as { real?: unknown; imag?: unknown } | null
+  return (
+      v !== null &&
+        typeof v === 'object' &&
+        Object.getPrototypeOf(v) === Object.prototype &&
+        typeof v.real === 'number' &&
+        typeof v.imag === 'number'
+    ) ?
+      128
+    : 0
+}
+
+// formatComplex formats the complex number value of bits bits as Go does:
+// its parts formatted as floats of half its size, the imaginary one always
+// signed, in parentheses.
+function formatComplex(
+  value: $.Complex,
+  verb: string,
+  prec: number,
+  bits: number,
+): string {
+  const re = $.formatFloat(value.real, verb, prec, bits / 2)
+  let im = $.formatFloat(value.imag, verb, prec, bits / 2)
+  if (!im.startsWith('-') && !im.startsWith('+')) {
+    im = '+' + im
+  }
+  return '(' + re + im + 'i)'
 }
 
 function hasGoTypeName(value: unknown): value is { __goType: string } {
@@ -621,6 +675,10 @@ function defaultFormatMaybe(
         allowMethods,
       )
     }
+    const cbits = complexBits(value, typeInfo)
+    if (cbits !== 0) {
+      return formatComplex(value, 'g', -1, cbits)
+    }
     if (
       info?.kind === $.TypeKind.Struct ||
       $.isStructObject(value) ||
diff --git a/tests/tests/complex_arithmetic/complex_arithmetic.go b/tests/tests/complex_arithmetic/complex_arithmetic.go
new file mode 100644
index 000000000..dc6e3869c
--- /dev/null
+++ b/tests/tests/complex_arithmetic/complex_arithmetic.go
@@ -0,0 +1,37 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+// pair returns two complex128 values that the compiler cannot fold.
+func pair() (complex128, complex128) { return 1 + 2i, 3 - 1i }
+
+func main() {
+	// The binary operators, with variable and constant operands.
+	a, b := pair()
+	fmt.Println(a+b, a-b, a*b, a/b, -a, +a)
+	fmt.Println(a*2, 2*a, a/2, a+1i, 1-a)
+	fmt.Println(a == b, a != b, a == 1+2i)
+	fmt.Printf("%v %.2f %e %g %+v\n", a, a, b, -b, struct{ C complex64 }{2i})
+
+	// The assignment operators.
+	c := a
+	c += b
+	c -= 1
+	c *= a
+	c /= 2i
+	fmt.Println(c)
+
+	// complex64 rounds to the precision of float32 parts.
+	var x, y complex64 = complex64(a), complex64(b)
+	fmt.Println(x+y, x*y, x/y, -x)
+	x *= y
+	fmt.Println(x, real(x), imag(x))
+
+	// Division by zero and by infinities follows the Go runtime.
+	var zero complex128
+	inf := complex(math.Inf(1), 0)
+	fmt.Println(a/zero, zero/zero, a/inf, inf/a)
+}
diff --git a/tests/tests/complex_arithmetic/complex_arithmetic.gs.ts b/tests/tests/complex_arithmetic/complex_arithmetic.gs.ts
new file mode 100644
index 000000000..9f8eeceec
--- /dev/null
+++ b/tests/tests/complex_arithmetic/complex_arithmetic.gs.ts
@@ -0,0 +1,43 @@
+// Generated file based on complex_arithmetic.go
+// Updated when compliance tests are re-run, DO NOT EDIT!
+
+import * as $ from "@goscript/builtin/index.js"
+import * as fmt from "@goscript/fmt/index.js"
+import * as math from "@goscript/math/index.js"
+
+export function pair(): [$.Complex, $.Complex] {
+	return [$.complex(1, 2), $.complex(3, -1)]
+}
+
+export async function main(): globalThis.Promise<void> {
+	// The binary operators, with variable and constant operands.
+	let [a, b] = pair()
+	fmt.Println($.basicInterfaceValue($.complexAdd(a, b), "complex128"), $.basicInterfaceValue($.complexSub(a, b), "complex128"), $.basicInterfaceValue($.complexMul(a, b), "complex128"), $.basicInterfaceValue($.complexDiv(a, b), "complex128"), $.basicInterfaceValue($.complexNeg(a), "complex128"), $.basicInterfaceValue(a, "complex128"))
+	fmt.Println($.basicInterfaceValue($.complexMul(a, $.complex(2, 0)), "complex128"), $.basicInterfaceValue($.complexMul($.complex(2, 0), a), "complex128"), $.basicInterfaceValue($.complexDiv(a, $.complex(2, 0)), "complex128"), $.basicInterfaceValue($.complexAdd(a, $.complex(0, 1)), "complex128"), $.basicInterfaceValue($.complexSub($.complex(1, 0), a), "complex128"))
+	fmt.Println($.arrayEqual(a, b), !$.arrayEqual(a, b), $.arrayEqual(a, ($.complex(1, 2))))
+	fmt.Printf("%v %.2f %e %g %+v\n", $.basicInterfaceValue(a, "complex128"), $.basicInterfaceValue(a, "complex128"), $.basicInterfaceValue(b, "complex128"), $.basicInterfaceValue($.complexNeg(b), "complex128"), $.interfaceValue($.anonymousStructValue({C: $.complex64($.complex(0, 2))}, { kind: $.TypeKind.Struct, methods: [], fields: [/* @__PURE__ */ $.structField("C", /* @__PURE__ */ $.basicType("complex64"), [0], 0, true)] }), "struct{C complex64}", { kind: $.TypeKind.Struct, methods: [], fields: [/* @__PURE__ */ $.structField("C", /* @__PURE__ */ $.basicType("complex64"), [0], 0, true)] }))
+
+	// The assignment operators.
+	let c = a
+	c = $.complexAdd(c, b)
+	c = $.complexSub(c, $.complex(1, 0))
+	c = $.complexMul(c, a)
+	c = $.complexDiv(c, $.complex(0, 2))
+	fmt.Println($.basicInterfaceValue(c, "complex128"))
+
+	// complex64 rounds to the precision of float32 parts.
+	let x: $.Complex = $.complex64($.complex64(a))
+	let y: $.Complex = $.complex64($.complex64(b))
+	fmt.Println($.basicInterfaceValue($.complex64($.complexAdd(x, y)), "complex64"), $.basicInterfaceValue($.complex64($.complexMul(x, y)), "complex64"), $.basicInterfaceValue($.complex64($.complexDiv(x, y)), "complex64"), $.basicInterfaceValue($.complex64($.complexNeg(x)), "complex64"))
+	x = $.complex64($.complexMul(x, $.complex64(y)))
+	fmt.Println($.basicInterfaceValue(x, "complex64"), $.basicInterfaceValue($.real($.complex64(x)), "float32"), $.basicInterfaceValue($.imag($.complex64(x)), "float32"))
+
+	// Division by zero and by infinities follows the Go runtime.
+	let zero: $.Complex = $.complex(0, 0)
+	let inf = $.complex(math.Inf(1), 0)
+	fmt.Println($.basicInterfaceValue($.complexDiv(a, zero), "complex128"), $.basicInterfaceValue($.complexDiv(zero, zero), "complex128"), $.basicInterfaceValue($.complexDiv(a, inf), "complex128"), $.basicInterfaceValue($.complexDiv(inf, a), "complex128"))
+}
+
+if ($.isMainScript(import.meta)) {
+	await main()
+}
diff --git a/tests/tests/complex_arithmetic/expected.log b/tests/tests/complex_arithmetic/expected.log
new file mode 100644
index 000000000..ae949d538
--- /dev/null
+++ b/tests/tests/complex_arithmetic/expected.log
@@ -0,0 +1,8 @@
+(4+1i) (-2+3i) (5+5i) (0.1+0.7000000000000001i) (-1-2i) (1+2i)
+(2+4i) (2+4i) (0.5+1i) (1+3i) (0-2i)
+false true true
+(1+2i) (1.00+2.00i) (3.000000e+00-1.000000e+00i) (-3+1i) {C:(0+2i)}
+(3.5-0.5i)
+(4+1i) (5+5i) (0.1+0.7i) (-1-2i)
+(5+5i) 5 5
+(+Inf+Infi) (NaN+NaNi) (0+0i) (+Inf-Infi)
diff --git a/tests/tests/complex_arithmetic/index.ts b/tests/tests/complex_arithmetic/index.ts
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/tests/complex_arithmetic/tsconfig.json b/tests/tests/complex_arithmetic/tsconfig.json
new file mode 100644
index 000000000..005935f57
--- /dev/null
+++ b/tests/tests/complex_arithmetic/tsconfig.json
@@ -0,0 +1,25 @@
+{
+  "compilerOptions": {
+    "paths": {
+      "*": [
+        "./*"
+      ],
+      "@go/github.com/s4wave/goscript/*": [
+        "../../../*"
+      ],
+      "@goscript/*": [
+        "../../../tests/tests/complex_arithmetic/run/output/@goscript/*",
+        "../../../gs/*",
+        "../../../tests/deps/*"
+      ],
+      "@goscript/github.com/s4wave/goscript/tests/tests/complex_arithmetic/*": [
+        "./*"
+      ]
+    }
+  },
+  "extends": "../../../tests/tsconfig.base.json",
+  "include": [
+    "complex_arithmetic.gs.ts",
+    "index.ts"
+  ]
+}