The ultrastructure of the pharynx of Cosmocerca ornata is described. The tissues of the corpus can be differentiated into alternating radial and marginal regions on the basis of (i) their relative position in the corpus, (ii) the structure of the muscle fibres and (iii) cytoplasmic inclusions. The radial fibres contain two sizes of filaments, whereas the marginal fibres contain only one type. It is suggested that the former group are contractile and the latter supportive in function. The cuticle lining the triradiate lumen of the pharynx is simple in structure, whereas that lining the marginal canals is more complex. The marginal canal cuticle contains three layers and the centre one is composed of two types of rings which alternate in a vertical plane. Ring A is in continuity with the sub-cuticular cytoplasm and ring B contains alternating, radially arranged blocks of differing density. It is suggested that this arrangement serves to maintain the marginal canal in a distended state whilst the central lumen is contracted. Teeth are present at the junction of the marginal canal and central lumen, and together with those of the pharyngeal bulb, serve to filter solid material from the fluid contents. The wall of the pharyngeal bulb contains muscles and gland cells.