Eimeria callospermophili is reported for the first time from the round-tailed ground squirrel, Xerospermophilus tereticaudus, and for the first time in Arizona, U.S.A. This coccidian was found in 4 of 10 (40%) round-tailed ground squirrels. Sporulated oocysts were spheroidal to subspheroidal, 20.4 × 17.8 µm (18–25 × 14–22 µm), with a length/width ratio of 1.1 (1.0–1.3), an oocyst residuum, and 0–2 polar granules. Sporocysts were ovoidal, 9.2 × 6.8 µm (7–10 × 6–8 µm), with a length/width ratio of 1.2 (1.1–1.4), a Stieda body, and a sporocyst residuum. The measurements and structural features of these oocysts were similar to those reported for E. callospermophili from other marmotine rodents.