Three patients with Mycobacterium marinum infection are described. Key elements in the diagnosis of this infection are a high index of suspicion, taking a history with an emphasis on exposure to tropical fish or other potential sources of M. marinum infection, and tissue biopsy for culture and histology. The microbiologist should be informed about the suspicion of M. marinum infection so that appropriate cultures can be performed. As M. marinum does not grow under routine culture conditions, the diagnosis is easily missed resulting in delayed treatment. The treatment is essentially antimicrobial therapy for the superficial lesions supplemented by an appropriate surgical debridement especially when deep structures are involved.