Examinations of 18 consecutive fecal samples collected at 6-hr intervals from infected rats showed that oocysts increased in length and width by approximately 40% from the beginning to the end of the patent period. Unsporulated oocysts were first observed in the 3rd fecal collection, 84 to 90 hr postinoculation (PI) and 100 of these measured 9.9 to 14.3 by 8.8 to 12.1 u (mean 11.7 by 10.1). Patency ended with the 15th fecal collection, 156 to 162 hr PI, and 100 unsporulated oocysts from this sample measured 14.3 to 17.6 by 13.2 to 15.4 ft (mean 16.3 by 14.2). Despite the size increase during patency, oocyst shape-index did not change. Sporulated oocysts did not differ significantly from unsporulated oocysts in length or width. The most significant increases in both length and width of the sporulated oocysts occurred 90 to 96 and 120 to 126 hr PI. During studies on interspecific interactions between coccidian species, I observed that oocysts of Eimeria separata seemed to become larger as the infection progressed. Increase in the size of coccidian oocysts, although apparently unusual, has been observed by some investigators (Becker et al., 1955, 1956; Cheissin, 1947; Cordero del Campillo, 1959; Fish, 1931; Jones, 1932). was interesting to note that Becker et al. (1932) stated, It is evident here also that the oocysts [of E. separata] exhibit no tendency toward a larger or smaller size as the infection progresses. Since my observations appeared to conflict with those of Becker et al., the following experiment was conducted in order to determine if there was an increase in the size of oocysts during the course of the infection. MATERIALS AND METHODS Experimental animals The hosts used in this study were 2 female, coccidia-free, SPF, Fischer 344 strain, inbred albino rats (National Laboratory Animal Company, Creve Coeur, Missouri). They were kept in a room with a relatively constant temperature (22 to 25 C) and were allowed food and water ad lib. Thus an attempt was made to standardize genetic, hormonal, age, and some environmental influences, all factors which may influence the course of an infection. The E. separata isolate was obtained by inoculating rats with fecal material from an infected wild Rattus norvegicus. This infected fecal material was Received for publication 23 February 1971. * Present address: Department of Biology, University of New Mexico, Albuquerque, New Mexico 87106. sent to me by Dr. J. V. Ernst, USDA Regional Parasite Research Laboratory, Auburn, Alabama. The specific status of this isolate was determined by measurement of 100 sporulated oocysts and comparison of observed characters with those from previously described species (Levine and Ivens, 1965). The species was further confirmed by observation of the living endogenous stages of the parasite in the cecum-colon using the technique of Marquardt (1966) and comparing these stages with those originally described by Roudabush (1937).