Swimming-pool granuloma and fish tank granuloma refer to the infections caused by Mycobacterium marinum. After having been discovered in salt water fish in Philadelphia Aquarium and described in 1926, this skin infection was first reported in humans in 1951. It developed in people who had swum in contaminated swimming pools. M. marinum is a non-tuberculous, atypical mycobacterium, which is found on plants, soil and fish in freshwater and salt water worldwide. Humans become infected usually after trauma and contact with an aquatic environment. Infection is limited to the skin and usually occurs in healthy individuals, but in immunocompromised patients the infection may disseminate or spread to the subcutis and bone. The lesions usually appear as solitary nodules or plaques that may lead to suppurative ulcers after 2-3 weeks of incubation. Occasionally, there may be sporotrichoid spread along lymphatics. Its diagnosis is frequently delayed, probably because the infection is very rare and a history of aquatic exposure, which is present in the majority of cases, is often overlooked. Common misdiagnoses include fungal and parasitic infection, cellulitis, verrucous tuberculosis of the skin, gout, rheumatoid arthritis, a foreign body and a skin tumour. We present a case of a 39-year-old Caucasian male with a 12-month history of a single erythematous tender nodule on the right dorsal aspect of the right hand. Histopathological examination revealed longstanding suppurated granulomatous inflammation. The infection was not responsive to several courses of antibiotics until we introduced doxycycline capsules as monotherapy which led to complete remission after 5 months.