diff --git a/internal/database/postgres/migrations/000125_lock_default_user_groups.down.sql b/internal/database/postgres/migrations/000125_lock_default_user_groups.down.sql
new file mode 100644
index 00000000..ca42c1f5
--- /dev/null
+++ b/internal/database/postgres/migrations/000125_lock_default_user_groups.down.sql
@@ -0,0 +1,6 @@
+UPDATE groups
+SET system_managed = FALSE
+WHERE id IN (
+    '00000000-0000-5000-8000-000000000005',
+    '00000000-0000-5000-8000-000000000006'
+);
diff --git a/internal/database/postgres/migrations/000125_lock_default_user_groups.up.sql b/internal/database/postgres/migrations/000125_lock_default_user_groups.up.sql
new file mode 100644
index 00000000..4faf9643
--- /dev/null
+++ b/internal/database/postgres/migrations/000125_lock_default_user_groups.up.sql
@@ -0,0 +1,12 @@
+-- Lock the seeded Standard Users and Read-Only Users groups (issue #168).
+--
+-- Owner decision 2026-10-16: these default groups are system-managed, so the
+-- #1737 guard (ErrSystemManagedGroup) refuses API edits and deletes, like
+-- Administrators, Purchaser and RI Exchanger. Locking is by id, so a group an
+-- admin already renamed or edited is locked as-is; permissions are untouched.
+UPDATE groups
+SET system_managed = TRUE
+WHERE id IN (
+    '00000000-0000-5000-8000-000000000005',  -- Standard Users (000057)
+    '00000000-0000-5000-8000-000000000006'   -- Read-Only Users (000057)
+);
diff --git a/internal/database/postgres/migrations/000125_lock_default_user_groups_test.go b/internal/database/postgres/migrations/000125_lock_default_user_groups_test.go
new file mode 100644
index 00000000..f64fc280
--- /dev/null
+++ b/internal/database/postgres/migrations/000125_lock_default_user_groups_test.go
@@ -0,0 +1,56 @@
+//go:build integration
+
+package migrations_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/LeanerCloud/cloud-commitments-platform/internal/database/postgres/migrations"
+	"github.com/LeanerCloud/cloud-commitments-platform/internal/database/postgres/testhelpers"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMigration_LockDefaultUserGroups(t *testing.T) {
+	ctx := context.Background()
+	container := testhelpers.RequirePostgresContainer(ctx, t)
+	t.Cleanup(func() { require.NoError(t, container.Cleanup(context.Background())) })
+	pool, path := container.DB.Pool(), getMigrationsPath()
+	ids := []string{
+		"00000000-0000-5000-8000-000000000005", // Standard Users
+		"00000000-0000-5000-8000-000000000006", // Read-Only Users
+	}
+	const otherID = "00000000-0000-5000-8000-000000000007" // Purchaser, already locked by its own seed
+
+	locked := func(id string) bool {
+		var v bool
+		require.NoError(t, pool.QueryRow(ctx, `SELECT system_managed FROM groups WHERE id=$1`, id).Scan(&v))
+		return v
+	}
+
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, path, 124))
+	for _, id := range ids {
+		require.False(t, locked(id), "before 000125 %s must be editable", id)
+	}
+	// An admin-renamed group is still locked by id, with its permissions kept.
+	_, err := pool.Exec(ctx, `UPDATE groups SET name='Renamed Standard' WHERE id=$1`, ids[0])
+	require.NoError(t, err)
+	var permsBefore string
+	require.NoError(t, pool.QueryRow(ctx, `SELECT permissions::text FROM groups WHERE id=$1`, ids[0]).Scan(&permsBefore))
+
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, path, 125))
+	for _, id := range ids {
+		require.True(t, locked(id), "after 000125 %s must be system-managed", id)
+	}
+	var name, permsAfter string
+	require.NoError(t, pool.QueryRow(ctx, `SELECT name, permissions::text FROM groups WHERE id=$1`, ids[0]).Scan(&name, &permsAfter))
+	require.Equal(t, "Renamed Standard", name)
+	require.Equal(t, permsBefore, permsAfter)
+	require.True(t, locked(otherID), "Purchaser must stay locked")
+
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, path, 124))
+	for _, id := range ids {
+		require.False(t, locked(id), "down must restore %s", id)
+	}
+	require.True(t, locked(otherID), "down must not unlock groups it did not lock")
+}