Verotoxin-producing Escherichia coli O111:NM were isolated from two five week old Holstein calves with dysentery. On necropsy both calves had pseudomembranous ileitis, mucohemorrhagic colitis and proctitis. Large numbers of E. coli O111:NM were isolated from the colon and lesions typical of attaching-and-effacing E. coli were evident. The isolates from both calves had identical biochemical reactions and antimicrobial resistance patterns. Oral inoculation of a four day old colostrum deprived calf with 1 x 10(10) organisms of E. coli O111:NM produced a mild, focal colitis with typical attachment and effacement lesions. We conclude that the strain of E. coli O111:NM isolated from the clinical cases has the ability to produce colitis characterized by attachment and effacement of the colonic mucosa.