The properties and taxonomic position of bacterial strains isolated from diseased specimens of cultured yellowtail and eels were examined. The isolates were gram-positive, short-chain-forming, catalase-negative, facultatively anaerobic cocci. Growth at 10 and 45 degrees C in 6.5% NaCl (pH 9.6) with 40% bile and in 0.1% methylene blue-milk were both positive. The isolates could be distinguished from other species of the genus Enterococcus by several biochemical characteristics and by Lancefield's group antigen. Guanine-plus-cytosine content of DNA was 44 mol% as determined by the thermal melting temperature. The value for DNA-DNA hybridization was sufficiently low to warrant distinguishing this species from reported. Enterococcus species. The name Enterococcus seriolicida is proposed. The type strain is YT-3 (=ATCC 49156).