A 33-year-old fish fancier developed a protracted skin infection that ultimately was found to be caused by Mycobacterium marinum. The organism was isolated from the lesion as well as from infected fish taken from his home aquarium. The lesion resolved after a six-week course of oral sulfamethoxazole and trimethoprim. Forty-four additional cases of culture-proved M marinum skin infections acquired from aquariums and reported in the English-language literature are reviewed. Almost universally, the lesions remained circumscribed and were either single nodular (14 patients) or multiple sporotrichoid (31 patients). Diagnosis was supported by acid-fast smears (15 patients) and isolation of the organism from skin lesions (43 patients) or from fish (two cases). In vitro studies, as well as clinical outcomes, suggest sulfamethoxazole-trimethoprim or ethambutol hydrochloride plus rifampin to be the drugs of choice.