An epizootic in seawater cage-reared large yellow croaker, Pseudesciaena crocea, in China from September to November 2003 was caused by a Nocardia sp. The cumulative mortality was 15% and diseased fish were 16 months old with total length from 20 to 30 cm. Multiple, white nodules, 0.1 - 0.3 cm in diameter, were scattered on the heart, spleen and kidney. The morphology of isolated bacteria from Lowenstein-Jensen medium （LJM） and trypfic soy agar （TSA） were Gram-positive, branching, bead-like or long, slender, filamentous rods. Experimental infection indicated that the isolated bacterium was pathogen responsible for the mortalities and the bacterium colonizes the fish via skin or gill, and intragastric exposure to the pathogen was less effective as a route of infection. Based on the symptom of diseased fish and the growth characteristics, morphological, physiological, and biological properties of the bacterium, the isolated were identified as Nocardia sp.