---

Background: Mycobacterium marinum is an atypical mycobacterium that causes a type of skin infection known as fish tank granuloma affecting people who are exposed to aquatic environments and manifests with erythematous, papulonodular or plaque-like lesions of the extremities. Case: A 15 year-old woman working in a tropical aquarium in Khomein referred with a non-tender nodule on the dorsum of fourth finger of left hand for 4 months. There were no specific findings in physical examination. Routine laboratory tests were normal. Skin biopsy was obtained which revealed suppurative granulomas, but staining of PAS and Ziel-Nelson were negative. The patient was treated with doxycycline 100 mg P.O. BID for 2 months with complete improvement of the lesions. Conclusion: In patients with a history of work in aquatic environments who manifest erythematous, papulonodular, or plaque-like lesions of the extremities should be checked in terms of fish tank granuloma and infection with mycobacterium marinum and skin biopsy and culture should be done for lesions.