Between January 1979 and December 1980, rotaviruses were detected in faecal samples from 122 (33.1%) of 369 diarrhoeic children less than six years old, living in Belém, Brazil. In 55 (45.1%) of the 122 rotavirus-positive specimens, no bacteria or parasite associated with gastro-enteritis was found. Salmonella typhimurium, Shigella and Escherichia coli were found in 92 (35%) of the 263 faecal specimens examined for bacterial pathogens. Rotaviruses were readily detected throughout the year, which may indicate no seasonal pattern of incidence in the Belém region. A comparison of the excretion of rotavirus was made between children of two socio-economic groups. Of 144 children from the higher level group, 63 (47.3%) excreted rotavirus, whereas only 12 (16.7%) of those from the lower level did so. Vomiting (68.0%) and fever (65.6%) were the main symptoms, other than diarrhoea, exhibited by the 122 rotavirus-positive children.