Two independent isolates of an unusual type of rotavirus were obtained from over 900 faecal samples obtained from piglets with diarrhoea. Genome profiles, containing eleven segments, were not characteristic of group A electropherotypes and corresponded to an atypical virus. The profiles were, however, distinct from those of other previously described atypical porcine rotaviruses. Serological comparisons showed that the two isolates were related, but unrelated to other known groups of porcine rotavirus. The new isolates were passaged in gnotobiotic piglets in which they caused mild clinical signs of enteritis. A survey of porcine sera from the United Kingdom indicated a widespread distribution of antibody to this virus in pigs aged 10 weeks and over.