The longitudinal musculature (taeniae) of the rabbit proximal colon was studied by phase contrast and electron microscopy. The taenia coli of the rabbit has several structural features which are not found in other visceral muscles of this or other species, and which, on the other hand, are common in certain vascular muscles. Among the characteristics of taenia coli are: highly corrugated muscle cell profiles (even when the muscle is fixed in a resting condition), very high surface-to-volume ratio (1 micron m2/0.5 micron m3), large percentage volume of the extracellular space (about 40%), absence of intramuscular septa, and very large number of elastic fibres, which run mainly longitudinally in broad meshworks parallel to the serosal surface. Several points of contact between elastic fibres and muscle cell membrane occur at all levels along the length of the cells, often involving 11-nm microfibrils; these contacts, and others involving collagen fibrils, are regarded as cell-to-stroma junctions. The taenia coli is virtually devoid of intramuscular blood vessels, including capillaries. The possibility that the lack of capillaries is related to the abundance of elastic material is discussed.