diff --git a/browser/Mmakefile b/browser/Mmakefile
index 4de054180..3acab9240 100644
--- a/browser/Mmakefile
+++ b/browser/Mmakefile
@@ -166,6 +166,9 @@ depend_aux: browse_test.depend declarative_test.depend
 # This directory contains source files for which the module name
 # does not match the file name, so smart recompilation won't work
 # without the Mercury.modules file.
+# XXX this .PHONY directive is required when building the source
+# distribution -- why?
+.PHONY: Mercury.modules
 Mercury.modules: MDB_FLAGS
 	$(MC) $(ALL_GRADEFLAGS) $(ALL_MCFLAGS) -f *.m
 
diff --git a/compiler/Mmakefile b/compiler/Mmakefile
index 8461b94f0..fda76ef5a 100644
--- a/compiler/Mmakefile
+++ b/compiler/Mmakefile
@@ -93,6 +93,9 @@ $(MC_PROG).depend: Mercury.modules COMP_FLAGS
 # This directory contains source files for which the module
 # name doesn't match the file name, so smart recompilation
 # won't work without the Mercury.modules file.
+# XXX this .PHONY directive is required when building the source
+# distribution -- why?
+.PHONY: Mercury.modules
 Mercury.modules: COMP_FLAGS
 	$(MC) $(ALL_GRADEFLAGS) $(ALL_MCFLAGS) -f *.m