Of 163 fecal samples collected between March 1997 and February 1999 from the plains vizcacha, Lagostomus maximus (Rodentia: Chinchillidae), 19 (12%) were found to be positive for coccidia. All species are from the genus Eimeria and 2 are described here as new. The third species is consistent with the description of Eimeria chinchillae De Vos and Van der Westhuizen, 1968. Sporulated oocysts of Eimeria lagostomi n. sp. are ellipsoidal to subspheroidal, 35.7 × 30.9 (26–36 × 30–41), with a length:width (L/W) ratio of 1.2 (1.0–1.3), without a micropyle (M) or polar granule (PG), but with an oocyst residuum (OR) comprised of a round, compact mass of many small granules. The sporocysts are lemon-shaped, 14.2 × 10.2 (9–11 × 11–16), with an L/W ratio of 1.4 (1.2–1.7) and have a Stieda body (SB) and sporocyst residuum (SR). Eimeria vizcacha n. sp. has sporulated oocysts that are subspheroidal, 26.4 × 23.4 (21–27 × 23–31), with an L/W ratio of 1.1 (1.1–1.2), lack an M and OR, but have 1–2 PGs. Sporocysts are elongate-ellipsoidal, 14.3–9.0 (8–10 × 13–15), with an L/W ratio of 1.6 (1.4–1.8), lack an SB, but have 2 SR; the first a small mass of granules lying between the sporozoites in the middle or at 1 end, and the second also of many small granules, always at the opposite end. Sporulated oocysts of the E. chinchillae-like organism are ellipsoidal to subspheroidal, 20.7 × 17.5 (15–22 × 17–24) with an L/W ratio of 1.2 (1.0–1.3), lack an M and OR, but have 0–3 PGs. Sporocysts are ellipsoidal, 10.7–6.9 (6–8 × 8–13) with an L/W ratio of 1.55 (1.3–1.8) and have an SB and SR.

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