Biologic, biochemical, and serologic characteristics of 79 halophilic vibrios isolated in the United States were studied and compared with 100 isolates of Vibrio parahaemolyticus obtained from patients with gastroenteritis. Sixty-three of the isolates were identified as V. parahaemolyticus. Eleven of these had combinations of 0 and K antigens that were previously unreported. They were 01:K5, 01:K33, 01:K37, 01:K44, 03:K25, 04:K37, 05:K3, and 07:K39. Whether or not this represents a true geographic variation in the sertotypes of this bacterium is unknown. The remaining 16 isolates studied were not V. parahaemolyticus. Six were Vibrio alginolyticus, and 10 were related marine vibrios.