It has been suggested that interstitial cells of Cajal (ICC) at the submucosal border of the colonic circular muscle are pacemaker cells. We studied smooth muscle cells and ICC at the submucosal surface of the circular muscle layer of the normal human colon. Resected, unaffected specimens from human colon were studied by light microscopy and transmission electron microscopy. Throughout the colon, the inner fourth of each circular-muscle lamella was covered with a layer of 2 to 15 muscle cells (ICMC) with a smaller diameter, more perinuclear organelles, and a greater glycogen content than the outer circular muscle cells. ICMC were interconnected by adherens junctions and close appositions. Small bundles of ICMC were present in the submucosa. ICC were identified in all regions of the colon (ascendens, transversum, and sigmoideum) at the submucosal border, in deeper parts of the submucosa in close contact with smooth muscle bundles as well as in the circular muscle and main septa. ICC had a continuous basal lamina, caveolae, dense bands, thin and intermediate filaments, dense bodies and a well-developed smooth endoplasmic reticulum. Mitochondria and granular endoplasmic reticulum were very abundant. Lipid droplets and glycogen granules were frequent. Thick (myosin) filaments were absent. Close contacts to nerves and gap junctions to other ICC or smooth muscle cells were exceptional. Fibroblast-like cells in the submucosa were rich in granular endoplasmic reticulum and intermediate filaments. They had few dense bands and caveolae. Mitochondria, smooth cisternae and glycogen granules were sparse, cytoplasmic dense bodies and a continuous basal lamina were lacking. Fibroblast-like cells were associated closely with collagen bundles and they had no close contacts with nerves, ICC or muscle cells. Throughout the normal human colon, submucosal ICC and ICMC are identified and distinguished from other cell types present. Their organization and cytology differ from that of other animal species. The ultrastructure of ICC and ICMC is compatible with important regulatory functions on the circular muscle in the entire human colon.