Vibrio cholerae serogroup O1 can be detected in the environment in a viable but nonculturable form, whereas V. cholerae non-O1 cells can be readily cultured during interepidemic periods in geographical regions where cholera is endemic. In the present study, pure cultures of V. cholerae non-O1 cells contained O1 cells when examined by immune-fluorescence microscopy. Laboratory microcosms were used to examine the outgrowth of the O1 cells in cultures of non-O1 V. cholerae. One O1 cell per 10(6) non-O1 cells could be detected by direct fluorescent-monoclonal antibody staining but only after incubation of the non-O1 culture for 48 h. Individual O1 cells were not detected in cultures incubated less than 48 h. Hybridization study, using a polymerase chain reaction (PCR) amplified fragment of the O-antigen of V. cholerae O1 as a probe, revealed the existence of a homologous gene in a microcosm sample of V. cholerae non-O1 containing serogroup-converted cells. The mechanism by which O1 cells can occur in cultures of non-O1 V. cholerae most likely resulted from spontaneous mutation of gene(s) encoding the O-somatic properties and (or) chemical, physical, or biological changes in the environment inducing expression or repression of the controlling gene(s). These findings have important implications for the epidemiology of cholera and the environmental source(s) of toxin producing V. cholerae O1.