Soon after its entry into the Indo-Pakistan subcontinent in 1964, cholera El Tor progressively replaced classical cholera. One of the probable reasons for this was found from laboratory studies of the interaction of the two choleragenic vibrios, V. cholerae and V. El Tor. It was observed, in both in vitro and in vivo experiments, that in mixed culture El Tor vibrios from cholera cases are capable of outgrowing and rapidly eliminating V. cholerae. The apathogenic El Tor strains from the Middle East countries differed significantly from the pathogenic type of El Tor strains isolated from cholera cases in that the former did not inhibit the growth of V. cholerae strains. The possible mechanism of interaction of the pathogenic El Tor vibrios and V. cholerae was studied in detail. Various possibilities, including higher rates of multiplication of El Tor vibrios, competition for nutrients, secretion of inhibitory substances and liberation of lethal bacteriophage or vibriocins by El Tor vibrios, were examined. Although it was not possible to establish the actual mechanism of this interaction, the possible biological effect on the epidemiology of cholera was evident.