Abstract.The Formosa snakehead,Channa maculata(Lacepède), and the largemouth bass,Micropterus salmoides(Lacepède), were given an intraperitoneal (i.p.) and/or intramuscular (i.m.) inoculation ofNocardia asteroidesisolated from cultured Formosa snakehead, Typical granulomatous lesions induced by the experimental fish were similar to the naturally infected cases. The Formosa snakehead showed 100% and 93% mortality at 14 and 35 days after i.m. inoculation with 8 mg and 0·8 mg of bacterial suspension, respectively. The largemouth bass was slightly resistant toN. asteroidesbut still had 95% mortality at 35 days after inoculation (i.m.) with 8 mg bacterial suspension. The bacteria were easily reisolated from spleen and kidney of the experimental fish. Many typical multifocal granulomata were found at the sites of injection and the internal organs such as liver, spleen, kidneys, pancreas, intestine, gills, eyes and brain. The result indicated that theN. asteroidesisolated from the field ease was pathogenic to Formosa snakehead and largemouth ba