Using a recently developed somatic O-antigen serogrouping scheme, 118 isolates of Aeromonas spp. were serogrouped, 68 of which were isolated from diarrhoea patients admitted to the Infectious Diseases Hospital and B.C. Roy Children's Hospital, Calcutta, from March 1987 to February 1988 and 50 randomly selected from an environmental study conducted during the same period. The isolates were typed into 23 different serogroups which represent 35.9% of the total 64 O-serogroups in the typing scheme. Only 12 strains were not typable by the available O-antisera. The dominant serogroups recovered from clinical and environmental isolates were O34, O16 and O11. It appears from this study that virulence of individual isolates is not linked to serological identity.