The authors have examined the ultrastructure of the rectum of infective-stage Wuchereria bancrofti by transmission electron microscopy. Our observations show that the rectum is divided into anterior and posterior segments. The cells of the anterior rectum appear to be derived from the microfilarial R (rectal) cells described by other authors. In both stages, these cells show voluminous nuclei, abundant mitochondria, and small cytoplasmic processes which contain fibrillar components. Amorphous material associated with these processes appears throughout the larval rectum and may protrude from the anus as the rectal plug. In the specimens examined, a patent lumen could not be traced completely through the anterior rectum. The posterior rectum has no counterpart in published accounts of microfilarial ultrastructure and probably arises during larval morphogenesis; it is lined with invaginated body cuticle, overlaid by a single layer of epithelial cells which may be of hypodermal origin.