IDIR broke out in the suckling rats of Wistar rat colony in August 1990. The suffering rats aged 10-14 days showed main clinical symptom of acute diarrhea. It was proved to be Rotavirus by electrophoresis of PAGE and negative stain EM. We observed the morbidity and mortality of infectious suckling rats in different birth order in a colony of 840 rats. The results of morbidity was reduced by birth order.