Mycobacterium marinum is the cause of opportunistic infections in man. Although its clinical presentation is usually cutaneous, osteoarticular infections are not rare and should be rapidly diagnosed. Orthopaedic surgeons may have to manage a patient with this mycobacterial infection and should be able to make this diagnosis based on information about the patient’s history and clinical criteria. Lesions develop from a skin wound, with a single nodule or a bright purplish-red patch with papules; they also may be inflamed or may abscess. Secondary lesions may develop as the disease progresses. Aquatic exposure is the most important factor to look for. We report three cases of this infection with a delayed diagnosis. Response to treatment and an absence of complications are correlated with an early diagnosis.