A clear difference has been observed between the classical Inaba V. cholerae and the El Tor Ogawa V. cholerae in relation to the ability to isolate the organism from the environment.An early attempt to utilize nightsoil sampling as a tool to measure the extent of infection in the community during an epidemic of classical Inaba cholera in Dacca, East Pakistan, in the spring and fall of 1968 proved unsuccessful. During an epidemic caused by both the classical Inaba and the El Tor Ogawa vibrios in Chittagong between July 1968 and March 1969 the reasons for this failure became apparent. In Dacca, only 2 isolations of classical Inaba were made from 9906 individual latrine and pooled communal nightsoil samples, whereas in Chittagong, from 62 588 similar samples in which 2 classical Inaba isolations were also made, there were 52 El Tor Ogawa isolations. In areas where cases due to both biotypes were occurring simultaneously, El Tor Ogawa vibrios were isolated 10 times more frequently than the classical Inaba.It remains unclear whether the differences observed between El Tor Ogawa and classical Inaba are related to the biotype or to the serotype of the organism, or to both. An extrapolation of nightsoil sampling, therefore, to the incidence and prevalence of infection in a community must consider both the biotype and the serotype.