diff --git a/asdf.lisp b/asdf.lisp
index 7042cb13a1c752e7f3c2ffe5b186fe1ec50ae98d..b76e5d27ecd254141c0e119b74a2ebfac6c55a9f 100644
--- a/asdf.lisp
+++ b/asdf.lisp
@@ -2339,6 +2339,8 @@ with a different configuration, so the configuration would be re-read then."
       ((:tree)
        (destructuring-bind (pathname) rest
          (collect-asd-subdirectories pathname :collect collect)))
+      ((:exclude)
+       (setf *default-exclusions* rest))
       ((:default-registry)
        (default-registry))
       ((:inherit-configuration)