We collected fecal samples from 9 dusky rice rats, Melanomys caliginosus (Rodentia, Cricetidae, Sigmodontinae), in a Biological Reserve in Costa Rica and found 8 (89%) to be infected with 2 Eimeria species which we describe here as new. Sporulated oocysts (n = 20) of the first, Eimeria melanomytis n. sp., are cylindroidal and measure 20.1 × 13.3 μm (18–23 × 13–15); micropyle and oocyst residuum are both absent, but a bilobular polar granule is present. Its sporocysts are ovoidal, 10.5 × 7.4 μm (10–13 × 6–8) with a small Stieda body, but both substieda and parastieda bodies are absent; a spheroidal sporocyst residuum is present, ∼5 μm wide. Sporulated oocysts (n = 20) of the second, Eimeria rebambensis n. sp., are subspheroidal, 21.2 × 17.0 μm (19–23 × 14–18); micropyle and oocyst residuum are both absent, but with a polar granule ∼2 μm wide. Sporocysts are elongate-ovoidal, 12.4 × 7.0 μm (11–14 × 6–9) with a distinct knob-like Stieda body, and a substieda body directly beneath it which is about twice as wide, but no parastieda body is present; the sporocyst residuum is an irregular mass composed of 8–10 globules scattered among the sporozoites, which are ∼10 × 5 μm, and have 1 refractile body at their wider end and a central nucleus. These are the first eimerians described from this rodent genus.

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