This paper describes an investigation on an epizootic of nocardiosis in female broodstock of striped mullet, Mugil cephalus (Linnaeus), cultured in central Taiwan during 1998. The nocardiosis present in the study was a chronic systematic infection in striped mullet. Several diseased fish showed sluggish swimming behavior and skin ulcers. Typical lesions were marked with white nodules and were scattered throughout the gills, liver, spleen, kidney, stomach, intestines, mesentery, and gonads. The most-severe lesions were found in the spleen and kidney.Ten bacterial strains were isolated from the moribund diseased fish and were demonstrated to be gram-positive, branching mycelia, whose fragmentation occurred in a zigzag pattern. The chemotype of the bacterial strains A212 and B222 was type IVA containing meso-DAP, arabinose, and galactose. The predominant menaquinone was MK-8(H4), and mycolic acids were present in the cellular composition. The results suggested that bacterial strains A212 and B222 were Nocardia seriolae.