A reovirus-like agent (rotavirus) was detected in 26 children (44%) when fecal specimens from 59 children with acute gastroenteritis were examined by EM. No rotavirus was detected in the feces of 49 children with other diseases. Sera from the acute and the convalescent phase from 40 children with acute gastroenteritis and from 18 other patients were examined for antibodies against a bovine rotavirus by an indirect fluorescent antibody test. Of the patients with gastroenteritis, 26 (65%) developed antibodies during their disease; other patients showed a rise in antibody titer. The specimens were submitted to the laboratory from hospitalized children from Jan. 1973-March 1975. Most of the cases of rotavirus gastroenteritis occurred during late autumn and early winter among children between 0.5 and 3 yr of age. EM is apparently a sensitive diagnostic technique during the acute phase of the disease, and serological testing with bovine rotavirus antigen serves as a useful diagnostic tool.