SYNOPSIS. Eimeria ochrogasteri n. sp. (Coccidia, Eimeriidae) from a prairie vole Microtus ochrogaster (Rodentia, Cricetidae) is described. This is the first recorded coccidium in prairie voles. Sporulated oocysts spherical to ellipsoidal, mean 24.0 by 20.5 μ. Oocyst wall double, outer layer thick, yellow-brown, deeply pitted, inner layer clear. Oocyst residuum varies from many small globules to a coalesced group of large and/or small globules. Polar granule present. Micropyle absent. Sporocysts ovoid with “capped” Stieda body, mean 12.3 by 8.2 μ. Sporocyst residuum present. Sporozoites average 14.9 by 2.9 μ with spherical anterior and oblong posterior refractile globules. This species was found in 1 of 71 voles from Weld county, Colorado.