The etiologic role of rotavirus in acute diarrhoeal illness in children under five years of age was studied over a period of one year in an urban slum community. Rotavirus was detected in 17.7 per cent of 376 children with diarrhoea and 4 per cent of 299 healthy children with maximum prevalence at 19-24 months of age with statistically comparable detection in 0-6 months of age. The overall prevalence was significantly higher in children below 2 yr as compared to those between 2-5 yr of age. Girls (24.1%) were more susceptible amongst the diarrhoeal group in comparison to boys (13.8%). Diarrhoea due to rotavirus was more prevalent during the cooler months (November-February) of the year and no correlation was observed with rainfall and relative humidity. Rotavirus as the sole pathogen was observed in 9 per cent children with diarrhoea and in the rest, association of Cryptosporidium (3%) was observed for the first time with this virus along with other known enteropathogenic bacteria and parasites, enterotoxigenic Escherichia coli being the commonest organism.