Island which had been designated as approved or prohibited by the National Shellfish Sanitation Program. Fecal coliforms counts, aerobic plate counts, and Vibrio parahaemolyticus densities were determined for the samples. Mean V. parahaemolyticus density was more than 100 times greater in oysters than in water, whereas density of fecal coliforms was approximately 10 times higher in oysters. Seasonal and geographical distributions of V. parahaemolyticus were related to water temperature, with highest densities in samples collected in the spring and the summer along the Gulf coast. The synthetic DNA probe for thermostable direct hemolysin hybridized with 2 of 50 isolates, 1 of which was positive by the Kanagawa test. Vibrio parahaemolyticus is an enteric pathogen transmitted to humans primarily through consumption of raw or mishandled seafood (2, 23). Like other members of the genus Vibrio, it is a gram-negative, halophilic bacterium that occurs naturally in estuarine environments (15). Its distribution is worldwide, but reported densities of V. parahaemolyticus in the environment and in seafood vary greatly according to season, location, sample type, fecal pollution, and analytical methodology (3, 5, 8, 16, 27). The public health significance of V. parahaemolyticus contamination in seafoods is debatable, since few environmental or seafood isolates produce the thermostable-direct hemolysin (TDH) or Kanagawa hemolysin associated with most (96.5%) pathogenic strains found among clinical isolates (12, 20, 21). However, a TDH-related hemolysin has been isolated from Kanagawa phenomenon-negative V. parahaemolyticus strains associated with gastroenteritis in Southeast Asia (13). In an evaluation (5) of four methods for the enumeration of V. parahaemolyticus in seawater and oysters, we found that the hydrophobic grid membrane filter method described by Entis and Boleszczuk (9) gave the highest density estimates. This method had been used to evaluate the effects of thermal stress on artificially contaminated seafoods but had not been used in environmental surveys. Past V. parahaemolyticus surveys were conducted regionally but did not compare distribution of the organism among the Pacific, Gulf, and Atlantic coasts. The present study reports the incidence of V. parahaemolyticus in U.S. oysters and seawater as determined by the hydrophobic grid membrane filter method. The National Shellfish Sanitation Program classification of shellfish-growing areas is based on fecal coliform densities in the overlying waters (14); however, a statistical evaluation is needed to correlate V. parahaemolyticus densities in shellfish with either fecal coliform or V. parahaemolyticus counts for the overlying waters. In shellfish, V. parahaemolyticus

Two membrane filter (MF) and two most-probable-number methods for enumerating Vibrio parahaemolyicus were compared. The MF methods used elevated-temperature incubations (41 and 42°C) and were more specific than the most-probable-number methods (conducted at 35°C). The MF method with a hydrophobic grid and a repair step was most effective.