A total of 371 fecal specimens from infants and children with acute gastroenteritis in Maizuru, Tokyo and Osaka, Japan from July 2002 to June 2003 were tested for the presence of diarrheal viruses by reverse transcription-polymerase chain reaction (RT-PCR), reverse passive hemagglutination (PRHA), RNA-polyacrylamide gel electrophoresis (PAGE), latex agglutination and sequence analysis methods. Among diarrheal viruses detected, group A rotavirus was the most prevalent (42.2%) followed by norovirus (28.9%), group C rotavirus (8.4%), sapovirus (6.7%), adenovirus (5.3%) and astrovirus (0.9%), respectively. There was the high rate (7.6%) of viral mixed infections. Sapovirus was classified into 6 genotypes (GI/1, GI/4, GI/5, GI/6 and GII/1 and one novel tentatively called GII/5). It is noteworthy that genogroup II sapovirus can be classified into 5 genotypes. Our findings confirmed the presence of many diarrheal viruses co-circulating among Japanese infants and children and showed the great genetic diversity among sapoviruses.