We report results of surveillance for cholera caused by Vibrio cholerae O13
9 from September 1992, when it was first identified, to December 1998. V. c
holerae O139 dominated as the causative agent of cholera in Calcutta during
1992-93 and 1996-97, while the O1 strains dominated during the rest of the
period. Dramatic shifts in patterns of resistance to cotrimoxazole, neomyc
in, and streptomycin were observed. Molecular epidemiologic studies showed
clonal diversity among the O139 strains and continuous emergence of new epi
demic clones, reflected by changes in the structure, organization, and loca
tion of the CTX prophages in the V. cholerae O139 chromosome.