diff --git a/src/compiler.c b/src/compiler.c
index d0d353e..e0a5eca 100644
--- a/src/compiler.c
+++ b/src/compiler.c
@@ -8,7 +8,6 @@
 #include "system.h"
 #include "knowledge.h"
 #include "symbol.h"
-#include "substitution.h"
 #include "compiler.h"
 #include "switches.h"
 #include "specialterm.h"
diff --git a/src/mgu.h b/src/mgu.h
index ded4e12..dbe509d 100644
--- a/src/mgu.h
+++ b/src/mgu.h
@@ -3,7 +3,6 @@
 
 #include "term.h"
 #include "termlist.h"
-#include "substitution.h"
 
 //! A special constant do denote failure.
 /**