Ketoconazole (Nizoral®, Janssen Pharmaceutica) a new systemic antimycotic was tested in dermatomycosis in cats and dogs. The daily dose (10 mg/kg body weight) was administered for 10 or 20 days without any other measures being taken. After the 20‐day treatments new hair growth was observed in 96.7 per cent of the cats and 89.9 percent of the dogs. Clinical cure was complete in 96.8 per cent of the cats and 90.5 per cent of the dogs.