diff --git a/Changes b/Changes
index b3eea44e1..06d184de7 100644
--- a/Changes
+++ b/Changes
@@ -3,6 +3,10 @@ Revision history for Verilator
 The contributors that suggested a given feature are shown in [].  [by ...]
 indicates the contributor was also the author of the fix; Thanks!
 
+* Verilator 3.65**
+
+**** Warn if flex is not installed.  [Ralf Karge]
+
 * Verilator 3.650 4/20/2007
 
 **   Add --compiler msvc option.  This is now required when Verilated code
diff --git a/configure.ac b/configure.ac
index 54b490df4..d1d862151 100644
--- a/configure.ac
+++ b/configure.ac
@@ -16,8 +16,17 @@ AC_PROG_CC
 AC_PROG_CXX
 AC_PROG_INSTALL
 AC_PATH_PROG(PERL,perl)
-AC_PATH_PROG(YACC,bison)
+if test "x$PERL" = "x" ; then
+   AC_MSG_ERROR([Cannot find "perl" in your PATH, please install it])
+fi
 AC_PATH_PROG(LEX,flex)
+if test "x$LEX" = "x" ; then
+   AC_MSG_ERROR([Cannot find "flex" in your PATH, please install it])
+fi
+AC_PATH_PROG(YACC,bison)
+if test "x$YACC" = "x" ; then
+   AC_MSG_ERROR([Cannot find "bison" in your PATH, please install it])
+fi
 
 dnl Checks for libraries.