diff --git a/cmd/ctwatch/main.go b/cmd/ctwatch/main.go
index 128f895..669fa72 100644
--- a/cmd/ctwatch/main.go
+++ b/cmd/ctwatch/main.go
@@ -15,8 +15,17 @@ var stateDir = flag.String("state_dir", cmd.DefaultStateDir("ctwatch"), "Directo
 func main() {
 	flag.Parse()
 
-	var domains []string
+	if flag.NArg() == 0 {
+		fmt.Fprintf(os.Stderr, "Usage: %s [flags] domain ...\n", os.Args[0])
+		fmt.Fprintf(os.Stderr, "\n")
+		fmt.Fprintf(os.Stderr, "To read domain list from stdin, use '-'. To monitor all domains, use '.'.\n")
+		fmt.Fprintf(os.Stderr, "See '%s -help' for a list of valid flags.\n", os.Args[0])
+		os.Exit(2)
+	}
+
+	var matcher ctwatch.Matcher
 	if flag.NArg() == 1 && flag.Arg(0) == "-" {
+		var domains []string
 		scanner := bufio.NewScanner(os.Stdin)
 		for scanner.Scan() {
 			domains = append(domains, scanner.Text())
@@ -25,15 +34,11 @@ func main() {
 			fmt.Fprintf(os.Stderr, "%s: Error reading standard input: %s\n", os.Args[0], err)
 			os.Exit(3)
 		}
-	} else {
-		domains = flag.Args()
-	}
-
-	var matcher ctwatch.Matcher
-	if len(domains) == 0 {
+		matcher = ctwatch.NewDomainMatcher(domains)
+	} else if flag.NArg() == 1 && flag.Arg(0) == "." { // "." as in root zone
 		matcher = ctwatch.MatchAll{}
 	} else {
-		matcher = ctwatch.NewDomainMatcher(domains)
+		matcher = ctwatch.NewDomainMatcher(flag.Args())
 	}
 
 	cmd.Main(*stateDir, matcher)