Enteropathogenic Escherichia coli (0111:K58:H-) was isolated from the small bowel of two infants with chronic diarrhea. Small bowel biopsy revealed focal adherence of the bacteria to the epithelium, accompanied by inflammation of the tissue. The isolates also adhered to tissue culture cells in densely packed aggregates. The pattern of adherence to tissue culture cells were found only among strains of serogroups 0111 and 0119 when 196 enteric E. coli isolates were tested. The pathogenic mechanism of enteropathogenic E. coli thus appears related to dense colonization of intestinal tissue. Adherent strains may be detected by tissue culture assay.