diff --git a/ioctl_inner.go b/ioctl_inner.go
index 272b50b..0ded8bc 100644
--- a/ioctl_inner.go
+++ b/ioctl_inner.go
@@ -1,4 +1,4 @@
-//go:build !windows && !solaris && !aix
+//go:build !windows && !solaris && !aix && !plan9
 // +build !windows,!solaris,!aix
 
 package pty
diff --git a/ioctl_unsupported.go b/ioctl_unsupported.go
index e17908d..a07a64d 100644
--- a/ioctl_unsupported.go
+++ b/ioctl_unsupported.go
@@ -1,5 +1,5 @@
-//go:build aix
-// +build aix
+//go:build aix || plan9
+// +build aix plan9
 
 package pty
 
diff --git a/start.go b/start.go
index 9b51635..781bb68 100644
--- a/start.go
+++ b/start.go
@@ -1,5 +1,5 @@
-//go:build !windows
-// +build !windows
+//go:build !windows && !plan9
+// +build !windows !plan9
 
 package pty
 
diff --git a/start_plan9.go b/start_plan9.go
new file mode 100644
index 0000000..61f5d7b
--- /dev/null
+++ b/start_plan9.go
@@ -0,0 +1,11 @@
+package pty
+
+import (
+	"os"
+	"os/exec"
+)
+
+// StartWithSize can be supported after a fashion on Plan 9, later.
+func StartWithSize(cmd *exec.Cmd, ws *Winsize) (*os.File, error) {
+	return nil, ErrUnsupported
+}
diff --git a/winsize_unix.go b/winsize_unix.go
index 8dbbcda..8fe007b 100644
--- a/winsize_unix.go
+++ b/winsize_unix.go
@@ -1,5 +1,5 @@
-//go:build !windows
-// +build !windows
+//go:build !windows && !plan9
+// +build !windows !plan9
 
 package pty
 
diff --git a/winsize_unsupported.go b/winsize_unsupported.go
index 0d21099..8b3eee4 100644
--- a/winsize_unsupported.go
+++ b/winsize_unsupported.go
@@ -1,5 +1,5 @@
-//go:build windows
-// +build windows
+//go:build windows || plan9
+// +build windows plan9
 
 package pty