This study was carried out to investigate the occurrence of potentially pathogenic species of vibrio in sea water and estuarine environments of the Caspian Sea in the Golestan province of Iran. A total of 127 water samples from coastal waters as well as from river and estuaries were collected and analyzed by culture, during April and September 2001. Following prompt centrifugation, the resuspended sediments were initially enriched in alkaline peptone water and subsequently plated onto selective media including thiosulphate-citrate-bile salts-sucrose. Presumptive vibrio colonies were isolated and identified to species level using a gallery of biochemical and differential tests. In this study, Vibrio vulnificus was the predominant species isolated from sea waters (41%), followed by Vibrio parahaemolyticus (33%), Vibrio alginolyticus (15%), Vibrio fischeri (5%), Vibrio natriegens (4%), Vibrio damsela (0.7%), and Vibrio harveyi (0.7%). Non-01 Vibrio cholerae was the most abundant vibrio species recovered from fresh surface waters (80% of the total isolates). Five Vibrio Cholerae 01 species isolated, all belonging to Ogawa serogroups, were also recovered from the estuaries. The results of this study demonstrated the presence of pathogenic vibrio species in Caspian sea and coastal waters. The potential sanitary risk associated with the presence of these bacteria in the aquatic environment emphasizes the necessity of long-term monitoring programs.