The relationship among (i) the local incidence of cholera, (ii) the prevalence in the aquatic environment of Vibrio cholerae, and (iii) bacterial viruses that attack potentially virulent O1 and O139 serogroup strains of this organism (cholera phages) was studied in Dhaka, Bangladesh. Over nearly a 3-year period, we found that significantly more environmental water samples contained either a phage or a phage-susceptible V. cholerae strain than both (P < 0.00001). The number of cholera patients varied seasonally during this period and frequently coincided with the presence of pathogenic V. cholerae strains in water samples that otherwise lacked detectable cholera phages. Interepidemic periods were characterized by water samples containing cholera phages but no viable bacteria. Our data support the conclusion that cholera phages can influence cholera seasonality and may also play a role in emergence of new V …