Ketoconazole, an antifungal imidazole derivative, was used successfully to treat Tricophyton mentagrophytes infection in a dog. The drug was administered orally (11 mg/kg of body weight, q 24 h) and continued for 90 days. Though ketoconazole is not licensed currently for veterinary purposes, it has been used successfully to treat dermatophyte infections as well as intermediate and deep fungal diseases in both dogs and cats. In this case, ketoconazole was found to be nontoxic and less expensive than griseofulvin in the treatment of dermatophytosis.