Two cases of aquarium-associated Mycobacterium marinum infections are described. Neither was cured by the initial therapy consisting of surgical excision followed by doxycycline treatment. Both strains of M. marinum were shown to be resistant to doxycycline. In one patient the lesions were subsequently healed with cotrimoxazole. The other patient, a 12-year-old girl with sporotrichoid spread of the lesions on the lower arm, failed to respond satisfactorily to a combination of rifampicin and ethambutol despite favourable effect in vitro of both drugs. Nine months after the initial treatment, the infection was finally cured with renewed surgery followed by additional chemotherapy with rifampicin and ethambutol. No signs of immunological disorder could be detected in the patient. Increased awareness of the possibility of tetracycline resistance of M. marinum is advocated. A combination of adequate chemotherapy and surgical intervention may be required at least in complicated cases.