Detection of a rotavirus in faeces of 789 calves developing diarrhoea gave positive results among 48% of the calves. The same investigation extended to 96 apparently healthy animals shows that 12.5% of the faeces contained rotavirus. It appears that all the last ones cannot be considered as healthy controls. Association of rotavirus and E. coli K99+ is found in 5% of sick animals less than 10 days old.