The prevalence of Vibrio cholerae contamination in river water derived from 20 sites of 18 rivers in Kanagawa, Japan, was investigated during a period from July to September, 1987, and from one of the 20 sites in August, 1988 and in February, 1989, V. cholerae non-O1 was found in all samples at concentrations of 0.9-->1,400 MPN/100 ml. Higher amounts of the organism were observed in the samples from estuaries. V. cholerae O1 was detected in samples collected in August, 1988 and in February, 1989 at concentrations of 150 MPN/100 ml and 1.5 MPN/100 ml, respectively. From 1989 to 1995, water samples were collected monthly from 10 sites of 10 rivers to detect V. cholerae. V. cholerae O1 and non-O1 were detected in 3.6% (30 of 840) and in 61.1% (513 of 840) in the water samples examined, respectively. Overall, V. cholerae was found in 62.9% (528 of 840). Both types, O1 and non-O1, of organisms were detected in 15 samples. These results indicated that river water was contaminated frequently with V. cholerae non-O1 and sporadically with V. cholerae O1 throughout the year. Only one strain of V. cholerae O1 out of 543 V. cholerae strains was found to be a producer of cholera toxin. During these studies, the selectivity of 3 media for V. cholerae O1 was evaluated, and PMT agar was found to be the best.