Five litters of piglets born within two days of each other, together with their dams, were investigated for faecal excretion of group A rotavirus antigen from birth to two months old. All the 50 piglets in these litters became infected with the virus between 19 and 35 days old. Rotavirus excretion was first seen in one litter which was housed with other litters not included in this study. Two days later, piglets of the second litter in another farrowing room began to excrete rotavirus, and then infection spread to the other three litters in the same room. Within each of these litters, one or two piglets were infected early and thereafter infection spread to other piglets. It took four to 10 days for rotavirus to infect every piglet within a litter, and 16 days in total before all piglets in the five litters were infected. No rotavirus antigen was demonstrated in faeces from sows during the investigation period.