Excretion patterns of fecal viruses were studied in a cohort of 51 rural Costa Rican children. The presence of rotavirus, adenovirus, coronavirus-like particles, and small round viruses was investigated by electron microscopy (EM) in 2,516 extracts of weekly fecal specimens. Rotavirus was in addition studied with ELISA. The incidence of diarrhea was 0.7 episodes per child-year. Rotavirus was the most common virus (0.53 infection/child-year), followed by adenovirus (0.46 infection/child-year), and coronavirus-like particles (0.24 infection/child-year). However, the pathogenicity of rotavirus and adenovirus was low: only 3 of 24 rotavirus infections and 2 of 21 adenovirus infections were associated with diarrheal illness (12.5% and 9.5%, respectively). Small round viruses were detected in 23 specimens, but could not be assigned to a particular group of viruses. Children who excreted coronavirus-like particles and small round viruses were asymptomatic. Typical Norwalk-like viruses, astrovirus or calicivirus were not encountered. Rural conditions, good hygiene and prolonged breast feeding may explain the reduced exposure and pathogenicity of viral enteropathogens in rural Costa Rica.