diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index 08d3f5f..3bc5cf4 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1533,6 +1533,14 @@
 conditionals are LightBlue for better distinction.
 
 
+FORTH						*forth.vim* *ft-forth-syntax*
+
+Files matching "*.fs" could be F# or Forth.  If the automatic detection
+doesn't work for you, or you don't edit F# at all, use this in your
+startup vimrc: >
+   :let filetype_fs = "forth"
+
+
 FORTRAN					*fortran.vim* *ft-fortran-syntax*
 
 Default highlighting and dialect ~