The existence of natural infection with attaching and effacing Escherichia coli (AEEC) in chicks was reported. Numerous Gram-negative bacilli were attached to the enterocyte in association with lesions characterized on a wavy appearance of the mucosal surface in the intestines of six chicks. Immunohistochemically, these bacteria reacted positively with antiserum to Escherichia coli (O 103). By electron microscopy, numerous colibacilli were seen to be closely attached to the surface membranes of the enterocytes. In regions of bacterial attachment, almost all cell microvilli were effaced, and some of the remaining ones were elongated and/or disoriented. Part of the cell membrane formed a cup invagination and pedestal-like protrusion associated with the attached bacteria. A concentration of electron-dense material was seen beneath the adherent organisms. Bacteriologically, numerous E. coli (O 103:H-) were isolated from the jejunal contents of two chicks and the E. coli did not produce verotoxin or enterotoxin. The characteristic lesions could also be induced in the cecal mucosa of young chicks experimentally inoculated with the isolated E. coli. This is the first report of natural infection in chicks with AEEC.