Cysts of Besnoitia jellisoni were found in 36 of 196 Dipodomys ordii and nine of 36 D. microps from Utah, and three of seven D. merriami from California. Only adult and young adult animals were infected. Twenty Peromyscus maniculatus collected in Utah and 19 Perognathus formosus collected in California were not infected. The cysts in the kangaroo rats appeared as white spherical bodies ranging in size from 329 to 586 Au. The cysts were found in the subcutaneous tissue, in the abdominal mesenteries, and in the connective tissue covering the abdominal and reproductive organs; cysts were also found within these organs. In many infected animals cysts were embedded in the bones of the skull; small pits, often completely perforating the bones, were associated with these cysts. No clinical signs associated with B. jellisoni infections were seen in kangaroo rats kept in captivity. No evidence of reduced longevity was observed in the animals found to be infected. Little is known as to the hosts, incidence, and pathogenicity of the species of Besnoitia. During an experimental study of coccidiosis in kangaroo rats in Utah, we encountered numerous cysts of B. jellisoni in the carcasses of the kangaroo rats used. In this paper we report our findings concerning the incidence and location of this parasite in kangaroo rats in the western United States; the negative results of examinations of two genera of wild mice are also included. MATERIALS AND METHODS From September 1966 through August 1967 we collected 232 kangaroo rats (196 Dipodomys ordii and 36 D. microps) and 20 Peromyscus maniculatus in an area approximately 8 miles long by 3 miles wide in the Curlew Valley, Box Elder County, Utah. The majority of the kangaroo rats were captured in a 1-square-mile location (Range 11W, Township 13N, Section 9) within this larger area. The methods used to collect, care for, and determine the species and age of the kangaroo rats are described elsewhere (Ernst, 1967; Ernst, Hammond, and Chobotar, 1968). The animals were examined at necropsy for the presence of Besnoitia cysts. Some died, or were killed when in a moribund condition, within a few days after being brought into the laboratory. The remaining animals were used to study the life cycle of a new Eimeria species, and were examined for Besnoitia after they had been in captivity for 1 to 6 months; a few were examined after a longer period of time. The specimens of P. maniculatus were collected Received for publication 2 February 1968. * Supported in part by National Science Foundation Research grant GB-5338 and by Public Health Service fellowships 1-F1-GM-29,830 and 1-F1-GM-31,285 from the Institute of General Medical Sciences. Published as Journal Paper No. 750, Utah Agricultural Experiment Station. i snap traps and examined within 24 hr of collection. Seven specimens of Dipodomys merriami and 19 specimens of Perognathus formosus were collected in snap traps by one of us (ECO) during the summer of 1967 in a small area in Eureka Valley, Inyo County, California. These animals were preserved in 10% formalin and shipped to Utah State University for examination. In examining the animals for Besnoitia, each was skinned, and the subcutaneous tissue, especially that of the back and skull, carefully observed for cysts. All of the internal organs, including the reproductive system, were also carefully examined. The heads of most of the infected kangaroo rats were removed and placed in a dermestid colony for cleaning; a few were preserved in 10% formalin. The carcasses and internal organs of some heavily infected animals were separately preserved in 10% formalin for later study. Ten cleaned skulls were examined in detail for the location of cysts, and the middle ears of four preserved heads were dissected. The tissues used for histological observations were treated by routine methods, and stained with Heidenhain's iron hematoxylin and Harris' hematoxylin and eosin Y.