Four lines of evidence suggest that the recent outbreak strains of Vibrio cholerae 0139 could have emerged from serogroup 0 1 strains typified by isolates MOl and M0477 described in this paper, which are neither truly classical nor truly El Tor in their biotype attributes. Firstly, like all 0139 isolates, these 0 1 strains, isolated in Madras during and before the 0139 outbreak, were resistant not only to polymyxin B but also to all biotypespecific choleraphages, i.e. classical phage (Dl49 and El Tor phages e4 and e5. Secondly, the restriction fragment pattern (RFP) polymorphism displayed by these strains for the cholera toxin (ctx) gene, were identical with those produced by 0139 isolates but were different from those of 0 1 type strains, namely V. cholerae 569B (classical) and V. cholerae MAK757 (El Tor). Thirdly, all the 0139 isolates and the two 0 1 isolates carried an identical large number of copies of cholera toxin gene in their chromosomes. Finally, the outer-membrane protein profiles of strains MO 1 and M0477 were identical to those of 0 139 isolates but were different from those displayed by strains 569B and MAK757.