Psychrotrophic Kanagawa-positive marine vibrios were isolated from soft-shelled clams (Mya arenaria) collected in Yaquina Bay, Oreg. The 235 vibrio isolates obtained were screened for Gram reaction and morphology, Kanagawa reaction on Wagastsuma agar, and response to selected biochemical tests. The vibrio selected for further study was grown in broth, and the hemolysin was precipitated from a cleared supernatant with solid ammonium sulfate. The hemolytic substance was partially purified by DEAE-cellulose and Sephadex G-100 column chromatography. The hemolysin contained protein essential for activity, was thermolabile, and was more active against rabbit erythrocytes at 37°C than at lower temperatures. The molecular weight was estimated at 55,000 by using a Sephadex G-100 column. Hemolytic activity was partially inactivated by gangliosides and lowered against horse erythrocytes. The hemolysin did not react with antibody prepared against vibriolysin from Vibrio parahaemolyticus WP-1 by the Ouchterlony method. The hemolysin was high in aspartic and glutamic acids and low in arginine and histidine. Electrophoresis on a sodium dodecyl sulfate-polyacrylamide gel gave three major bands. The hemolysin from a psychrotrophic vibrio and the hemolytic exotoxin of V. parahaemolyticus had some similar and dissimilar characteristics. The possibility that a Vibrio sp. other than V. parahaemolyticus might serve as the reservoir for the Kanagawa phenotype is discussed.