RDEC-1 is a strain of Escherichia coli that, in rabbits, attaches to intestinal mucosal epithelial cells bereft of microvillar borders and causes diarrhea by an unknown mechanism. The stages of attachment of RDEC-1 bacteria to mucosal epithelial cells were examined using high-voltage electron microscopy of thick (0.5-micrometers) sections of ileum and cecum of rabbits with diarrhea. The tissues were stained with ruthenium red or antisera to strain RDEC-1 OK antigens. Micrographs, including stereopairs, demonstrated several stages of bacterial attachment. Bacteria were attached to the glycocalyxes of epithelial cell microvilli and to pedestal-like extrusions of the surfaces of epithelial cells lacking microvilli. Structures consistent with bacterial pili were rarely visualized. Attachment to microvilli appeared to be a result of the interaction of polysaccharides of the microvillar glycocalyx and the K antigen of the bacterial capsule.