diff --git a/cmd/ctwatch/main.go b/cmd/ctwatch/main.go
index fd1c20a..6605ede 100644
--- a/cmd/ctwatch/main.go
+++ b/cmd/ctwatch/main.go
@@ -7,6 +7,8 @@ import (
 	"bufio"
 	"strings"
 
+	"golang.org/x/net/idna"
+
 	"src.agwa.name/ctwatch"
 	"src.agwa.name/ctwatch/ct"
 	"src.agwa.name/ctwatch/cmd"
@@ -24,17 +26,38 @@ var stateDir = flag.String("state_dir", DefaultStateDir(), "Directory for storin
 var watchDomains []string
 var watchDomainSuffixes []string
 
-func setWatchDomains (domains []string) {
+func addWatchDomain (domain string) {
+	watchDomains = append(watchDomains, strings.ToLower(domain))
+	watchDomainSuffixes = append(watchDomainSuffixes, "." + strings.ToLower(domain))
+}
+
+func setWatchDomains (domains []string) error {
 	for _, domain := range domains {
 		if domain == "." { // "." as in root zone (matches everything)
 			watchDomains = []string{}
 			watchDomainSuffixes = []string{""}
 			break
 		} else {
-			watchDomains = append(watchDomains, strings.ToLower(domain))
-			watchDomainSuffixes = append(watchDomainSuffixes, "." + strings.ToLower(domain))
+			addWatchDomain(domain)
+
+			asciiDomain, err := idna.ToASCII(domain)
+			if err != nil {
+				return fmt.Errorf("Invalid domain `%s': %s", domain, err)
+			}
+			if asciiDomain != domain {
+				addWatchDomain(asciiDomain)
+			}
+
+			unicodeDomain, err := idna.ToUnicode(domain)
+			if err != nil {
+				return fmt.Errorf("Invalid domain `%s': %s", domain, err)
+			}
+			if unicodeDomain != domain {
+				addWatchDomain(unicodeDomain)
+			}
 		}
 	}
+	return nil
 }
 
 func dnsNameMatches (dnsName string) bool {
@@ -105,9 +128,15 @@ func main() {
 			fmt.Fprintf(os.Stderr, "%s: Error reading standard input: %s\n", os.Args[0], err)
 			os.Exit(1)
 		}
-		setWatchDomains(domains)
+		if err := setWatchDomains(domains); err != nil {
+			fmt.Fprintf(os.Stderr, "%s: %s\n", os.Args[0], err)
+			os.Exit(1)
+		}
 	} else {
-		setWatchDomains(flag.Args())
+		if err := setWatchDomains(flag.Args()); err != nil {
+			fmt.Fprintf(os.Stderr, "%s: %s\n", os.Args[0], err)
+			os.Exit(1)
+		}
 	}
 
 	cmd.Main(*stateDir, processEntry)