The pejerrey, Odonthestes bonariensis, given intramuscular injections of Mycobacterium sp., isolated from pond-cultured pejerrey, developed a condition similar to the natural infection. The fish showed 100% mortality at 14 and 30 days after 10 and 1 mg/ml of bacterial suspension was injected, respectively. The bacterium could be easily re-isolated from the kidney of dead fish. In all injected fish, many typical focal granulomas were found in the site of injection and in the examined internal organs. These data proved that the bacterium was certainly a pathogen of pejerrey.