diff --git a/src/testdir/README.txt b/src/testdir/README.txt
index 76396fe..d639146 100644
--- a/src/testdir/README.txt
+++ b/src/testdir/README.txt
@@ -25,6 +25,8 @@
 bit faster, because Vim doesn't have to be started, one Vim instance runs many
 tests.
 
+At 4), to run a test in GUI, add "GUI_FLAG=-g" to the make command.
+
 
 What you can use (see test_assert.vim for an example):