The coccidian parasite Eimeria vilasi is reported for the first time from the round-tailed ground squirrel, Xerospermophilus tereticaudus. This coccidian was found in 9 of 94 (10%) round-tailed ground squirrels captured in Arizona. Sporulated oocysts were ovoidal, 20.1 × 18.1 (18–25 × 14–22) μm, with a length:width ratio of 1.1 (1.1–1.2). Sporocysts were ellipsoidal, 10.4 × 6.2 (8–13 × 5–7) μm, with a length:width ratio of 1.7 (1.3–2.2). The measurements and structural features of these oocysts were similar to those reported for E. vilasi from other marmotine rodents. Eimeria callospermophili was also recovered from 58 of 94 (62%) round-tailed ground squirrels.