We collected fecal samples from 9 dusky rice rats, Melanomys caliginosus (Rodentia, Cricetidae, Sigmodontinae), in aBiological Reserve in Costa Rica and found 8 (89%) to be infected with 2 Eimeria species which we describe here as new. Sporulatedoocysts (n=20) of the first, Eimeria melanomytis n. sp., are cylindroidal and measure 20.1x13.3 um (18–23x13–15); micropyle andoocyst residuum are both absent, but a bilobular polar granule is present. Its sporocysts are ovoidal, 10.5x7.4 um (10–13x6–8) with asmall Stieda body, but both substieda and parastieda bodies are absent; a spheroidal sporocyst residuum is present, ~5 lm wide.Sporulated oocysts (n=20) of the second, Eimeria rebambensis n. sp., are subspheroidal, 21.2x17.0 um (19–23x14–18); micropyleand oocyst residuum are both absent, but with a polar granule~2 lm wide. Sporocysts are elongate-ovoidal, 12.4x7.0 um (11–14x6– 9) with a distinct knob-like Stieda body, and a substieda body directly beneath it which is about twice as wide, but no parastieda bodyis present; the sporocyst residuum is an irregular mass composed of 8–10 globules scattered among the sporozoites, which are~10x5lm, and have 1 refractile body at their wider end and a central nucleus. These are the first eimerians described from this rodent genus.
