A strain of Escherichia coli (RDEC-1) has been described that in rabbits colonizes the intestine; adheres to mucosal epithelial cells of the ileum, cecum, and colon; and causes diarrhea by an unknown mechanism. This study attempted to determine the location of the bacteria in the rabbit intestine during the unexplained six- to seven-day interval between bacterial inoculation and onset of diarrhea. Specimens of ileum, cecum, colon, ileal Peyer's patches, sacculus rotundus, and appendix from control rabbits and from rabbits killed at intervals after inoculation with RDEC-1 bacteria were examined by light and direct fluorescence microscopy. Bacteria in large numbers attach to the tips of the Peyer's patch lymphoid follicles by 24 hr, but they did not attached to ileal, cecal, or colonic mucosa until three days after inoculation. The lag time between inoculation and onset of diarrhea was probably due to the need for the bacteria first to attach to and then colonize the Peyer's patch lymphoid follicles. The intestinal mucosa was probably colonized by bacteria shed from the Peyer's patches.