Recently, we reported the occurrence of seroconversion from Vibrio cholerae non-O1 to V. cholerae O1, but little is known about the environmental and physiological factors influencing seroconversion. We investigated effects of temperature (4, 25, and 35 degrees C) and salinity ( < 0.05 and 10%0.), as well as the stage of growth of cells, on serogroup conversion. Seroconversion of V. cholerae occurred under various environmental conditions. However, the rate of seroconversion in natural water ( < 0.5% salinity) and synthetic seawater microcosms (10%0 salinity), employing cells harvested from stationary phase culture, was approximately 2 logs higher than cells harvested from cultures in the logarithmic phase (i.e., 10(5) versus 10(3) per 10(10) cells. Thus, the physiological state of the cells, and to a lesser degree, temperature and salinity, is an important factor in the conversion of V. cholerae from non-O1 to O1 serogroup.