A rotavirus isolated from a field outbreak of diarrhoea in artificially reared piglets was purified, filtered and administered orally to gnotobiotic and conventional piglets. Four successive passages of the virus in gnotobiotic piglets produced severe diarrhoea within 20 to 24 hours of administration. The diarrhoea lasted several days causing dehydration, emaciation, loss of body weight and some deaths. Virus was demonstrated in the faeces of the infected piglets by electron microscopy. Conventionally reared piglets developed little or no diarrhoea when given virus, whereas artificially reared piglets developed moderate to severe diarrhoea which lasted from 3 to 8 days with some deaths. No clinical disease was obvious in surviving piglets following challenge with the virus 10 or 17 days after initial infections.