Some rotaviruses from calves, piglets, and lambs were detected by electron microscopic examination of faeces but not by an enzyme-linked immunosorbent assay which relies on detection of group antigen. On further examination by polyacrylamide gel electrophoresis, these viruses had 11 segments of dsRNA, as had typical rotaviruses, but arranged in atypical patterns. From humans, three rotaviruses with atypical electrophoretypes were also detected. Gnotobiotic animals were infected with atypical calf, piglet and lamb rotaviruses, and used to provide antigen and antiserum for an immunofluorescent comparison of these rotaviruses with conventional rotaviruses and other previously described atypical rotaviruses from piglets and chickens. Two atypical rotaviruses from humans failed to infect gnotobiotic piglets. The atypical rotaviruses could be tentatively categorized into two groups serologically distinct from each other and from conventional rotaviruses, and these distinctions were consistent with electrophoretypes . The atypical chicken rotavirus may form a fourth distinct group. These findings are consistent with the hypothesis that rotaviruses belong to at least four separate groups definable by serology and electrophoretype .