I was studying for my USMLE exams when I came across this so I thought I would take a break and share it. M. marinum is widely distributed in water and causes chronic cutaneous infection when an open cutaneous lesion is exposed to a colonized water source. Most infections are due to hand or upper-extremity exposure to fish tanks, and some are due to shellfish or marine exposures. Swimming pools are no longer a common source of infection because of current chlorination standards. M. marinum grows optimally at 30°C—a lower temperature than is optimal for most pathogenic mycobacteria. After a median incubation period of 21 days (30 days in 35% of cases), a granulomatous or ulcerating skin lesion develops at the site of entry, with subsequent sporotrichoid spread in many cases. In some patients, especially those with serious underlying disease and those receiving immunosuppressive therapy, infection may extend to deeper structures, producing tenosynovitis or osteomyelitis. The diagnosis is established by mycobacterial culture of a biopsied lesion or by demonstration of granulomas or AFB in a biopsy sample from a patient with a compatible exposure history. Treatment consists of the combination of clarithromycin and ethambutol; the regimen is given for 1–2 months after resolution of lesions—typically 3–4 months in total. Surgical debridement may be necessary in extensive or deep disease; however, routine incision and drainage are not helpful. Rifampin should be added in cases of osteomyelitis. Persons with occupational or avocational exposure to fish tanks or salt water should wear waterproof gloves to prevent infection of open cutaneous lesions.