By electron microscopy survey of acute gastroenteritis of children in Matsuyama, rotaviruses were detected in 561 of 2479 fecal samples obtained between October, 1984 and September, 1988, in which 60 atypical and 259 typical rotaviruses, so far tested, were detected by polyacrylamide gel electrophoresis (PAGE) of viral RNA. Out of 60 atypical rotaviruses, 5 were observed in 1985, 7 in 1986, none in 1987 and 48 in 1988. These atypical rotaviruses were morphologically indistinguishable from typical ones and showed very similar RNA migration patterns of PAGE to those of group C rotavirus. Furthermore, one atypical virus (86-542) reacted with antiserum against a porcine group C rotavirus in immune electron microscopy, while it did not react with anti-group A rotavirus serum. On the other hand, hyperimmune guinea pig antiserum against 86-542 reacted with a couple of atypical rotavirus including viruses isolated in 1988, so far tested, but did not react with any of the typical viruses. These findings showed that atypical rotaviruses which were endemic in Matsuyama city in 1988 were defined as group C rotavirus. Moreover, these group C rotaviruses showed two kind of RNA migration patterns in PAGE, which clearly discriminated the virus were isolated before and after 1987. Epidemiological features of group C rotavirus were as follows. Children from whom group C rotavirus was isolated were older than those from of whom group A rotavirus was isolated. The epidemic season was in February through April, compared to December through March in group A virus. Retrospective seroepidemiological study by immune adherence hemagglutination test (IAHA) using the purified 86-542 virus as antigen indicated that 15 of 78 sera of children obtained in 1971 already possessed IAHA antibody against group C rotavirus.