We examined the relationship between serotype and enterotoxin production in 109 enterotoxigenic Escherichia coli strains isolated from 109 patients with severe cholera-like diarrhea in Dacca, Bangladesh. Of 69 strains producing both heat-labile and heat-stable toxins, 59 (86%) belonged to the one of four O serogroups, and 56 (81%) of these strains belonged to one of six O:K:H serotypes. In contrast, 34 strains producing only heat-stable toxin were distributed among 15 O serogroups, and six strains producing only heat-labile toxin were distributed among six O serogroups. Twelve strains producing heat-labile and heat-stable toxins and five strains producing heat-stable toxin were found which had the same serotype (O78:K-:H12) and biotype. It appears that at least in one geographic setting E. coli strains producing both heat-labile and heat-stable toxins are more restricted in their O groups and O:K:H serotypes than E. coli that produce only heat-stable toxin and that certain serobiotypes may commonly include strains which produce both toxin types.