The extent of contamination of a freshwater lake with l/ibrio cholerae O139 Bengal and the toxigenicity of all the V. cholerae isolates recovered during the period of the study were examined during and after an explosive outbreak of O139 cholera in Calcutta. Strains biochemically characterized as V. cholerae could be isolated throughout the period of study examined from the freshwater lake samples. Most probable number of V. cholerae belonging to the O139 serogroup in surface waters was 3 to 4 per 100 ml during major part of the study but isolation of this serogroup from sediment and plankton samples was infrequent. Of the total of 150 strains recovered, 23 (15.3%) agglutinated with the O139 antiserum while the remaining belonged to the non-O1 non-O139 serogroups. None of the strains agglutinated with the O1 antiserum. All the 23 strains of V. cholerae O139 produced cholera toxin while 7.9% of the 127 non-O1 non-O139 strains also produced cholera toxin. Resistance to ampicillin, furazolidone and streptomycin was encountered among strains belonging to both V. cholerae O139 and V. cholerae non-O1 non-O139 strains, but the percentage of resistant strains in the former was much higher than in the latter. During this cholera epidemic, possibly due to the introduction of large numbers of toxigenic V. cholerae such as the O139 serogroup, there was an increase in the numbers of toxigenic vibrios among the innocuous aquatic residents. This presumably occurred through genetic exchange and, if substantiated, could play an important role in the re-emergence of epidemics.