We identified 5 patients who had cutaneous lesions with cultures that yielded Mycobacterium marinum. It was discovered that all 5 patients had a home aquarium, and infection was preceded by trauma to the hand. However, the association between the development of the infection and exposure of the trauma site to the aquarium was not initially established until repeated questioning was performed. Skin biopsies or incision and drainage were performed for all patients, and the diagnosis was established by culture of the specimens. The mean time from initial presentation to diagnosis and initiation of appropriate treatment was 91 days (range, 21-245 days). Prolonged therapy for 2 to 6 months was necessary for resolution of the infection.