diff --git a/Makefile b/Makefile
index 99126dfe..69335c8f 100644
--- a/Makefile
+++ b/Makefile
@@ -95,10 +95,13 @@ update-gomod:
 	GOPROXY=direct go get -d github.com/wader/gojq@fq
 	go mod tidy
 
+# Usage: make fuzz # fuzz all foramts
+# Usage: make fuzz GROUP=mp4 # fuzz a group (each format is a group also)
 # TODO: as decode recovers panic and "repanics" unrecoverable errors this is a bit hacky at the moment
-# fuzz code is not suppose to print to stderr so log to file
 # Retrigger:
+# try to decode crsash with all formats:
 # cat format/testdata/fuzz/FuzzFormats/... | go run dev/fuzzbytes.go | go run . -d raw '. as $b | formats | keys[] as $f | $b | decode($f)'
+# convert crash into raw bytes:
 # cat format/testdata/fuzz/FuzzFormats/... | go run dev/fuzzbytes.go | fq -d raw 'tobytes | tobase64'
 # fq -n '"..." | frombase64 | ...'
 .PHONY: fuzz
diff --git a/format/fuzz_test.go b/format/fuzz_test.go
index e51952e3..783b3226 100644
--- a/format/fuzz_test.go
+++ b/format/fuzz_test.go
@@ -66,7 +66,7 @@ func (ft *fuzzTest) Readline(opts interp.ReadlineOpts) (string, error) {
 
 func FuzzFormats(f *testing.F) {
 	if os.Getenv("FUZZTEST") == "" {
-		f.Skip("run with FUZZTEST=1 do fuzz")
+		f.Skip("run with FUZZTEST=1 to fuzz")
 	}
 
 	i := 0
@@ -103,7 +103,18 @@ func FuzzFormats(f *testing.F) {
 	}
 
 	gi := 0
-	g := interp.DefaultRegistry.MustAll()
+	var g decode.Group
+
+	if n := os.Getenv("GROUP"); n != "" {
+		var err error
+		g, err = interp.DefaultRegistry.FormatGroup(n)
+		if err != nil {
+			f.Fatal(err)
+		}
+		f.Logf("GROUP=%s", n)
+	} else {
+		g = interp.DefaultRegistry.MustAll()
+	}
 
 	f.Fuzz(func(t *testing.T, b []byte) {
 		fz := &fuzzTest{b: b, f: g[gi]}