The occurrence of mycobacteriosis caused by Mycobacterium marinum in a commercial breeding farm of bullfrogs (Rana catesbeiana) in Rio de Janeiro, Brazil is described. Ten animals presented skin lesions on the head and extremities. These and 38 other asymptomatic adult animals from various tanks were killed and at necropsy disseminated granulomatous lesions were observed in the 10 clinically affected animals and in 16 (42.1%) of the asymptomatic frogs. Acid-fast bacilli were observed in all smears of the 10 symptomatic frogs and in all but one from the 16 asymptomatic animals with visceral lesions. Ten samples from the 25 positive animals were randomly selected for culture which yielded four isolates of fast-growing (<7 days) mycobacteria. Those purified isolates were characterised by biochemical traditional means as M. marinum. Identification of the strains was confirmed using reverse-phase high-performance liquid chromatography and a polymerase chain reaction (PCR) restriction enzyme analysis assay. It is suggested that M. marinum is an important agent of granulomatous disease in bullfrogs and that infected animals, even when asymptomatic, could act as reservoirs spreading the disease and contaminating other frogs in the farm.