Eighteen Rocky Mountain bighorn sheep in Montana were examined for parasites at necropsy. Twelve came from the National Bison Range, five from Wildhorse Island (Flathead Lake), and one from the Sun River area. The parasites recovered were: Cysticercus tenuicollis, Wyominia tetoni, Marshallagia marshalli, Ostertagia circumcincta, 0. lyrata, 0. occidentalis, 0. ostertagi, Cooperia oncophora, C. surnabada, Nematodirus archari, N. davtiani, N. helvetianus, N. lanceolatus, N. spathiger, Trichostrongylus sp. (unidentified females), Protostrongylus rushi, Dermacentor albipictus, and D. venustus. Gastrointestinal nematodes recovered from the National Bison Range sheep ranged in number from 275 to 5,300 per host and those from the Wildhorse Island sheep from 670 to 3,510. The species recovered from the National Bison Range sheep were totally different from those recovered from the Wildhorse Island and Sun River sheep. The parasites recovered represent several new host and distribution records. Included in this report is a checklist of 51 species from Ovis canadensis, based on the present report, unpublished records of specimens in the U. S. National Museum Helminthological Collection, and from the literature. Seventy per cent of these 51 species are known parasites of domestic sheep and 35% of cattle in North America. During a study of the Rocky Mountain bighorn sheep, Ovis canadensis canadensis Shaw, 1804, by the staffs of the Department of Zoology and the Montana Cooperative Wildlife Research Unit of the University of Montana, parasites were recovered from 18 animals. Twelve rams were obtained by P. L. Wright and V. B. May from the National Bison Range at Moiese, Montana, through the cooperation of John Schwartz and C. J. Henry, former managers of the Range. Five sheep were obtained from Wildhorse Island on Flathead Lake, Lake County, Montana, and one pregnant ewe was obtained from the Sun River area in Teton County, Montana, with the assistance of personnel of the Montana State Game and Fish Department. The Sun River ewe was trapped at Scattering Springs. It was killed and necropsied at the Rocky Mountain Laboratory, Hamilton, Montana, by W. Hadlow. Some of the parasites recovered from the 18 sheep have been the subject of previous reports; namely, lungworms (Forrester and Senger, 1964), Bovicola jellisoni (Emerson, 1962), and Oestrus ovis (Capelle, 1966). The remaining parasites collected are reported herein, together with a checklist of the external and internal parasites of Ovis canadensis. Received for publication 18 July 1966. MATERIALS AND METHODS One ram was killed and necropsied each month for 12 consecutive months at the National Bison Range. The first ram was necropsied in May 1958, and the 12th in April 1959. The rams ranged in age from 2 to 7 years. They were first examined for ectoparasites, then the lungs, liver, omasum, abomasum, and small intestines were removed and taken to the laboratory for examination. The general method of recovering and estimating total numbers of gastrointestinal worms was similar to the procedure used by Porter (1942), with the exception that the fine plant debris in each container of organ contents and washings was reduced by means of a process of repeated suspensions in water, sedimentations, and decantations prior to taking samples. For most sheep, the total worm counts were determined from various small samples of the washings from the abomasums and small intestines of the sheep. In some instances, however, all the washings from the gastrointestinal tracts were examined and the total counts were made directly. Sheep 13, 14, and 15 from Wildhorse Island (Table II) were found dead or were accidentally killed in the field. They were not subjected to as thorough an examination for gastrointestinal parasites as were the other sheep from Wildhorse Island and the National Bison Range (Table I), and their worm counts are probably incomplete. Only qualitative determinations were made on the parasites recovered from sheep 18 from Sun River (Table II), as the procedures used in collecting these specimens are unknown. Fecal samples were collected from sheep 1 to 11 (Table I) from the National Bison Range, but not from the other seven sheep, and the nematode