To clarify the environmental distribution of Vibrio vulnificus, sea water, sea mud, and oysters were examined at 13 sites, i.e. 4 sites in the Tokyo Bay (eastern Japan) and 9 sites (5 sites for oysters) in Tokushima Prefecture (western Japan). 1. V. vulnificus was isolated from 80 (54.8%) of the 146 samples of sea water examined. It was isolated from 19 (41.3%) of the 46 samples from western Japan and 61 (61.0%) of the 100 samples from eastern Japan. 2. It was isolated from 40 (40.8%) of the 98 samples of sea mud obtained in eastern Japan. 3. It was isolated from 655 (30.3%) of the 2,165 samples of oysters. They were 30 (9.7%) of 309 samples from western Japan and 625 (33.7%) of 1,856 samples from eastern Japan. 4. The density of V. vulnificus was 0.3-1.1 x 10(6) MPN/L in seawater, 0.3-1.1 x 10(5) MPN/100 g in sea mud, and 0.3-1.1 x 10(7) MPN/100 g in oysters. 5. Seasonally, V. vulnificus was isolated from 44 (6.2%) of the 713 samples in spring, 450 (72.6%) of the 620 samples in summer, 264 (51.8%) of the 510 samples in fall, and 17 (3.0%) of the 56 samples in winter. Thus, the isolation rates of V. vulnificus from sea water and oysters tended to be higher in eastern Japan than in western Japan and to be highest in summer, then, in fall.