Oysters (Crassostrea gigas) originating from the southern coast of the State of São Paulo-Brazil were analyzed for Vibrio species. Most Probable Numbers (MPN/100 g) were obtained for Vibrio alginolyticus (Vibrio parahaemolyticus (Vibrio fluvialis (Vibrio cholerae non 01 (Vibrio furnissii (Vibrio mimicus (Vibrio vulnificus (V. alginolyticus (81%), followed by V. parahaemolyticus (77%), V. cholerae non 01 (31 %), V. fluvialis (27%), V. furnissii (19%), V. mimicus (12%), and V. vulnificus (12%). Forty-eight percent of the isolates tested were positive for enterotoxins in the rabbit ileal loop (RIL) test and 11.1% in the suckling mice test. Vibrio parahaemolyticus (1.1 %) was positive in the Kanagawa test. Vibrio vulnificus (25%) showed lethality in young adult mice. During the field inspection it was observed that generally the conditions of storage were not adequate as 65% of the samples were maintained in temperatures ranging from 25 to 40°C, 19% were frozen, 12% refrigerated and 4% was immersed in contaminated seawater. These results emphasize the great potential for food poisoning by inadequately preserved seafood, and the necessity to upgrade the standards for food quality assessment.