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Eimeria species are frequently found in water buffalo (Bubalus bubalis) in Brazil. Here, we report those Eimeria spp. that infect buffalos during their first year of life. Fresh fecal samples were examined from 2 groups (1 group/yr for 2 yr, 2000-2002), each with 18 water buffalo calves (both sexes), from birth through 12 months of age, in Selvíria, MS, Brazil. Five oocyst morphotypes were observed, i.e., Eimeria ellipsoidalis and Eimeria zuernii, both previously described from water buffalo, and 3 other morphotypes consistent with descriptions of known Eimeria spp. from Artiodactyla hosts, but originally described from other genera than those in which we found them (referred to here as Eimeria species 1-3). Our results showed that buffalo calves started shedding oocysts in their feces between 6-29 days of age, with the highest concentration ranging from 188-292 oocysts/g of feces. The 3 unnamed oocyst morphotypes in the calf feces resembled E. auburnensis (Eimeria sp. 3), E. cylindrica (Eimeria sp. 1), and E. subspherica (Eimeria sp. 2). The most prevalent species were Eimeria sp. 1 and E. ellipsoidalis, which dominated in the youngest animals (6 to 133 days old). Eimeria zuernii oocysts, in contrast, were found only in low numbers in the feces of older calves (208 to 283 days old). Calves were infected more frequently during the rainy season (September to January) in both years, but cows were negative for Eimeria spp., whenever feces were collected (spring, winter, autumn, or summer seasons).