diff --git a/src/leiningen/jar.clj b/src/leiningen/jar.clj
index e5aec284..e515ab60 100644
--- a/src/leiningen/jar.clj
+++ b/src/leiningen/jar.clj
@@ -133,9 +133,17 @@
 (defmethod copy-to-jar :bytes [project jar-os acc spec]
   (when-not (some #(re-find % (:path spec)) (:jar-exclusions project))
     (.putNextEntry jar-os (JarEntry. (:path spec)))
-    (io/copy (ByteArrayInputStream. (:bytes spec)) jar-os))
+    (let [bytes (if (string? (:bytes spec))
+                  (.getBytes (:bytes spec))
+                  (:bytes spec))]
+      (io/copy (ByteArrayInputStream. bytes) jar-os)))
   (conj acc (:path spec)))
 
+(defmethod copy-to-jar :fn [project jar-os acc spec]
+  (let [f (eval (:fn spec))
+        dynamic-spec (f project)]
+    (copy-to-jar project jar-os acc dynamic-spec)))
+
 (defn write-jar [project out-file filespecs]
   (with-open [jar-os (-> out-file
                          (FileOutputStream.)
diff --git a/test/leiningen/test/jar.clj b/test/leiningen/test/jar.clj
index 4cee03d6..a55dc97d 100644
--- a/test/leiningen/test/jar.clj
+++ b/test/leiningen/test/jar.clj
@@ -23,9 +23,11 @@
   (let [jar-file (JarFile. (jar sample-project))
         manifest (manifest-map (.getManifest jar-file))
         bin (slurp (.getInputStream jar-file (.getEntry jar-file "bin/nom")))
+        bin (slurp (.getInputStream jar-file (.getEntry jar-file "bytes.clj")))
         bat (slurp (.getInputStream jar-file (.getEntry jar-file
                                                         "bin/nom.bat")))]
     (is (= "bin/nom" (manifest "Leiningen-shell-wrapper")))
+    (is (= [:bytes "are" 'nomnomnom] (read-string bytes)))
     (is (re-find #"org/clojure/clojure/1\.1\.0/" bin))
     (is (re-find #"org\\clojure\\clojure\\1\.1\.0" bat))
     (is (re-find #"MAIN=\"nom\.nom\.nom\"" bin))
diff --git a/test_projects/sample/project.clj b/test_projects/sample/project.clj
index cb393682..07256b40 100644
--- a/test_projects/sample/project.clj
+++ b/test_projects/sample/project.clj
@@ -15,6 +15,9 @@
   :shell-wrapper {:main nom.nom.nom
                   :bin "bin/nom"}
   :jar-exclusions [#"^META-INF"]
+  :filespecs [{:type :fn :fn (fn [p] {:type :bytes :path "bytes.clj"
+                                     :bytes (str "[:bytes \"are\" "
+                                                 (:name p) "]")})}]
   :test-selectors {:integration :integration
                    :default (complement :integration)
                    :random (fn [_] (> (rand) ~(float 1/2)))