Between May 1, 1976 through May 14, 1977, feces from 1,056 five-day-old babies in newborn nurseries were examined by electron microscopy, of which 343 (32.5%) contained rotavirus. Rotaviruses were prevalent throughout the study period, but infection reached its peak during the winter months of December 1976 and January 1977, at which time 46% and 49% of babies, respectively, were excreting rotaviruses. Infection occurred substantially less frequently among breast-fed babies. Further, infected breast-fed babies excreted less virus than those who were bottle fed. Viral particles in feces of breast-fed babies frequently appeared in clumps, which usually contained complete smooth viral particles. Analysis of feeding charts in one of the wards revealed that infection was usually asymptomatic because only 15 of 189 (8%) babies had enteritis. None of these babies had symptoms of sufficient severity to necessitate treatment.