Records for cestodes and acanthocephalans resulting from an endoparasite survey of Peruvian and Colombian monkeys and marmosets are presented and discussed. Thirty-four of 65 animals harbored Prosthenorchis elegans or P. spirula; 11 animals were hosts for Atriotaenia megastoma, Raillietina (R.) trinitatae, or Hymenolepis cebidarum. New host records noted in this paper are as follows: P. spirula-Cebuella pygmaea; P. elegansTamarinus nigricollis, Cebuella pygmaea, Saimiri boliviensis; Hymenolepis cebidarum-Tamarinus nigricollis; Atriotaenia megastoma-Tamarinus nigricollis, Saimiri sciurea, Saimiri boliviensis. Previous host records for these helminths are reviewed and discussed. Many scientific names recorded in the literature for hosts of these parasites are synonyms of one another or have been superseded as a result of nomenclatural changes; an attempt is made to bring some order into the confused host record for several of the parasites. Host-parasite patterns are related to host diet and to probable modes of transmission. These patterns are shown to be consistent with the suspected transmission routes. The parasitologic record also provides data bearing on the natural diet of some of the primate hosts. The early papers in this series on endoparasites of Neotropical primates have been based on a survey of newly imported Peruvian and Colombian monkeys and marmosets carried out in San Francisco from 1960 to 1962. The methods and importing procedures have been discussed in several of the previous papers (Dunn and Lambrecht, 1963a, b; Dunn, Lambrecht, and du Plessis, 1963). In the present contribution some records are supplied for cestodes and for acanthocephalans of the genus Prosthenorchis, and present knowledge of the host-parasite patterns for these helminths in South and Central American primates is reviewed. The records presented below are based on the results of dissections of 65 animals of nine species. The hosts, countries of origin, and numbers examined are listed in Received for publication 14 June 1963. * The George Williams Hooper Foundation and Division of Parasitology, Department of Medicine, San Francisco Medical Center, University of California. Current address: Medical Zoology Laboratory, Institute for Medical Research, Kuala Lumpur, Federation of Malaya. This study was supported in part by U. S. Public Health Service Grant Al 04189-02 from the ICMRT Program, Office of International Research, National Institutes of Health. This paper is No. 6 in a series on the endoparasites of Neotropical primates. Table I. Common names for the hosts under consideration are as follows: Tamarinus nigricollis (Spix, 1823)-black-and-red tamarin; Oed pomidas oedipus (L., 1758)-pinche marmoset; Cebuella pygmaea (Spix, 1823)-pygmy marmoset; Callicebus cupreus (Spix, 1823) -red titi monkey; Aotes trivirgatus Humboldt, 1811-night-monkey; Saimiri sciurea (L., 1758)-common squirrel-monkey; Saimiri boliviensis d'Orbigny, 1834-black-headed squirrel-monkey; Lagothrix cana (Geoffroy) 1812Amazonian woolly monkey (in earlier papers of this series this animal was referred to as L. infumata); Ateles paniscus (L., 1758)black spider-monkey. THE GENUS Prosthenorchis