A bacterial-like tubercular disease broke out in reared snakehead, Ophiocephalus argus Cantor, in Xiaoshan in Zhejiang Province in June, 2006. Pure bacteria was isolated from liver and kidney in diseased fish and numbered W060622. Experimental infection indicated that the isolated bacterium was the pathogen responsible for the mortalities. Cultures from liver and kidney were incubated for 4－6 days at 28 ℃. Weakly pigmented yellowish adherent colonies 1－3 mm in diameter appeared as a heavy, almost pure growth on TSA. The bacteria were Gram-positive, aerobic, weakly acid-fast, bead-like or long, slender, and filamentous rods observed in virtue of microscope and electron microscope. The organism was found to be catalase-positive, oxidase-negative, urea-produced, and could reduce nitrate and aesculin but not degrade adenine, xanthine, hypoxanthine, tyrosine, gelatin, elastin, starch and casein. It could utilized the citrate as sole carbon source but not mannitol, arabinose and sorbitol. The traditional physiological and biochemical methods showed that most of the phenotypes of strain W060622 were similar to Nocardia. A partial sequence of 16S rRNA gene of the bacteria was determined following PCR amplification and sequencing. The GenBank Accession No. is EF192033. The sequence was aligned with those available for nocardiae and phylogenetic trees inferred using N-J method. The unrooted evolutionary tree shows that strain W060622 forms a monophyletic clade with Nocardia seriolae JCM 3360 T (Z36925). This relationship was highlighted in the analyses by the high nucleotide similarity value (99.9%) and the high bootstrap value (1000) based on the neighbour-joining method. Thus, the pathogenic organism was identified as Nocardia seriolea. This is the first discovery on Nocardia sp.-infected snakehead in aquaculture. Fish tubercular diseases were reported causing by the pathogen of Corynebacterium sp., Mycobacterium sp., Rickettsia-like organism, Nocardia sp., et al. We could differentiate them through the morphology, stain, physiological and Biochemical characters. A kidney with nodular lesions was made ultrathin sections and observed under transmission electron microscope and no other prokaryotes were found except bacteria of nocardia. Based on the high morbidity and mortality and the difficulty of being detected, we consider the pathology, immunology, detection and medication of nocardiosis in snakehead warrants further investigation.