A case of a sporotrichoid cutaneous infection caused by Mycobacterium marinum is reported. A 53- year-old male patient presented with red, partly purulent nodular lesions on the back of his left hand, forearm, and upper medial arm that had developed consecutively during the past 4 weeks. A mycobacterial infection with M. marinum was confirmed by molecular methods in a lesional skin biopsy. The patient was treated systemically with rifampicin (750 mg/day) and clarithromycine (1,000 mg/day), and topically with sulmycin (gentamicin sulfate). After 12 weeks of treatment the nodules regressed, leaving behind erythematous patches. M. marinum is a waterborne mycobacterium that commonly infects fish and amphibians worldwide. Transmissions to humans occur occasionally, in most cases as a granulomatous infection localized to the skin, typically following minor trauma to the hands. For this reason, infections are especially common among aquarium keepers.