The largemouth bass Micropterus salmoides Lacepede was given an intramuscular (IM) or intraperitoneal (IP) injection of Nocardia asteroides isolated from a diseased cultured largemouth bass, and typical granulomatous lesions similar to those of the natural infection were induced. The experimental fish showed 100% mortality 14 or 21 days after IP or IM injection of 8 mg of bacteria, and 28 days after IM injection of 0.8 mg of bacteria. The bacterium could be easily reisolated from the liver and kidney of dead fish. Many typical focal granulomata developed at the site of injection and in organs such as liver, spleen, kidney, eye, brain, pancreas, gill, stomach, intestine, and ovary. The result shows that the bacterium is clearly pathogenic to the largemouth bass.