A pathogenic vibrio, strain S3y was isolated from an outbreak of vibriosis in grouper (Epinephelus malabaricus) fry in Kaohsiung, Southern Taiwan in November 1991. It was identified as Vibrio alginolyticus on the basis of a number of biochemical tests. The bacterium was therefore identified as Vibrio, a Gram-negative, straight rod, motile, oxidase and catalase positive, swarming on TSA plate, fermentative and produced yellow colonies on TCBS agar. It did not utilise sodium citrate, but it was gelatinase positive and sensitive to the vibriostatic agent, 0/129. It could grow on TSA with up to 11% NaCl. Its G+C ratio was 47%. The minimum lethal dose of strain S3y was 500 cfu/g fish body weight in the grouper (Epinephelus malabaricus). The same bacterial species could be reisolated from the kidney of moribund fish following bacterial challenge. The extracellular products (ECP) of S3y were lethal to fish with a minimum lethal dose of 0.52 microgram/g fish body weight. A 34 kDa protease was purified from the ECP of S3y and demonstrated to be a toxin by intraperitoneal injection in the grouper. The minimum lethal dose of the purified protease was 0.17 microgram/g fish body weight. Intraperitoneal injection of the bacteria, ECP or the 34 kDa protease all caused slight exophthalmia with corneal opaqueness in moribund and dead fish. The results suggested that the 34 kDa protease might play an important role in the pathology of vibriosis caused by this bacterium.