diff --git a/doc/bison.1 b/doc/bison.1
index 0cd949ee..958bb398 100644
--- a/doc/bison.1
+++ b/doc/bison.1
@@ -328,6 +328,17 @@ will be removed, because it is incompatible with the POSIX.2 standard.
 /usr/local/lib/bison.simple	simple parser
 .br
 /usr/local/lib/bison.hairy	complicated parser
+.SH "ENVIRONMENT VARIABLES"
+.TP
+.SM BISON_SIMPLE
+If this is set, it specifies the location in which the
+.B bison.simple
+parser can be found.
+.TP
+.SM BISON_HAIRY
+If this is set, it specifies the location in which the
+.B bison.hairy
+parser can be found.
 .SH SEE ALSO
 .IR yacc (1)
 .br