Summary Preliminary data have been presented on a new and economically important nematode disease of captive monkeys. The parasite is described, and considered to be identical with Protospirura muricola, which is normally parasitic in the stomach of rats. Three monkeys (Cebus capucinus, Ateles dariensis, and Aotus zonalis) are recorded as new hosts for this parasite. Most of the fatalities from protospiruriasis have occurred among the Cebus monkeys, whose dietary habits are especially favorable to the acquisition of heavy worm burdens. Normally, the worms live in the esophagus and stomach; rarely they were encountered in the duodenum and body cavity. Injury to the host is the result of obstruction, tissue invasion, pressure action, and secondary infection. The cockroach, Leucophaea maderae (F.), serves as intermediate host. The infective larvae are contained in cysts which are found mainly in the thorax around the crop and at the bases of the large muscles of the first pair of legs. The morphology of this larva is described. A reduction in cockroach population was suggested as a preventive measure.