diff --git a/test/misc/Makefile b/test/misc/Makefile
index 1d6b2b9d2..cda0c789a 100644
--- a/test/misc/Makefile
+++ b/test/misc/Makefile
@@ -50,6 +50,11 @@ TESTS += $(foreach option,$(OPTIONS),$(SOURCES:%.c=$(WORKDIR)/%.$(option).65c02.
 
 all: $(TESTS)
 
+# The same input file is processed with different cl65 args,
+# but cl65 uses the input file name to make the temp file name,
+# and they stomp each other.
+.NOTPARALLEL:
+
 $(WORKDIR):
 	$(call MKDIR,$(WORKDIR))
 
diff --git a/test/val/Makefile b/test/val/Makefile
index df1d314e4..417c0b6c8 100644
--- a/test/val/Makefile
+++ b/test/val/Makefile
@@ -41,6 +41,11 @@ TESTS += $(foreach option,$(OPTIONS),$(SOURCES:%.c=$(WORKDIR)/%.$(option).65c02.
 
 all: $(TESTS)
 
+# The same input file is processed with different cl65 args,
+# but cl65 uses the input file name to make the temp file name,
+# and they stomp each other.
+.NOTPARALLEL:
+
 $(WORKDIR):
 	$(call MKDIR,$(WORKDIR))