A 19-month-old Holstein female manifested muco-hemorrhagic diarrhea. At necropsy, the colon contained a large amount of bloody material mixed with mucus and there was diffuse petechial hemorrhage on the mucosal surface. Histologic and electron microscopic examination of the colon revealed the characteristic lesion of attaching and effacing Escherichia coli (AEEC) infection. The organisms were associated with mucosal lesions and stained positively by an immunoperoxidase technique using antiserum to E. coli O group 15. Diarrhea caused by AEEC was recognized in this adult cow.