Mycobacterium marinum is a waterborne mycobacterium that commonly infects fish and amphibians worldwide. Infection in humans occurs occasionally, in most cases as a granulomatous infection localized in the skin, typically following minor trauma on the hands. For this reason, infection is especially common among aquarium keepers. Such local infection may-though infrequently-spread to tendon sheaths or joints. Disseminated disease, which is rare, can occur in immunosuppressed patients. In order to obtain a definitive diagnosis, culture and histopathological examination of biopsies from skin or other tissues are recommended. Infections sometimes heal spontaneously, but drug treatment is usually necessary for several months in order to cure the infection. Doxycycline or clarithromycin is used most commonly, although in severe cases, a combination of rifampicin and ethambutol is recommended.