In a study of field material and a survey conducted by the authors, typical signs of colibacillosis of 6-to-12-week-old poults included sudden onset, listlessness, rales, and high mortality. Signs persisted for about 2 weeks and were often followed by a low incidence of lameness caused by Escherichia coli. Gross lesions included enlarged and congested spleens and livers, and dilated discolored black or purple duodenal loops. Microscopic lesions included splenic and hepatic congestion. In some birds (freshly killed and fixed immediately), the epithelium at the tips of the duodenal villi was sloughing, but in other birds the villi were intact and normal in appearance. Splenic enlargement, the presence of intranuclear splenic inclusions similar to those found in hemorrhagic enteritis (HE), and the isolation of HE virus from some of the field spleens all indicated that inapparent HE infection often occurs at approximately the same time as this type of colibacillosis. It is therefore believed that HE infection often exacerbates colibacillosis of older poults.