During an epidemic of cholera we simultaneously cultured water from 30 important ponds, tanks and rivers of Dhaka city, to determine the role of surface water as a reservoir of Vibrio cholerae and nonagglutinating (NAG) vibrios and whether their presence or absence can be used as an indicator of a cholera epidemic in the community. Out of 4.016 samples 1216 (30.3%) were positive for NAG vibrios and one (0.025%) for Vibrio cholerae. Ponds showed a higher rate of NAG isolation (31.6%) than did rivers (21.9%). NAG group II constituted 87% and group V 10% of all positives. There were mild-to-moderate seasonal variations in isolation rates. The NAG isolation rate from water was related to the incidence of hospitalized NAG diarrhoea cases, but not to simultaneous cholera cases. Surface water does not appear to be an efficient natural reservoir of Vibrio cholerae, as it is for NAG vibrios. The absence of V. cholerae or presence of NAG vibrios in public surface water sources has no relationship with the extent of cholera cases present in a community.