Fecal samples from 43 broiler and breeder farms in the Entre Rios and Buenos Aires districts of Argentina were examined for Coccidia. Thirty-eight samples were positive by microscopic examination after salt flotation, with counts of 138 to 415,800 oocysts per gram of feces. Samples were aerated for 2 days and inoculated into 21-day-old chicks for determination of prepatent period. Eimeria praecox was conclusively identified in 56% of the samples by producing typical oocysts (17 x 21 microns) in the feces by 83-90 hr postinoculation (PI). Eimeria mitis was present in 67% of the samples, producing typical oocysts (14 x 15.5 microns) before 95 hr PI. Typical oocysts of Eimeria maxima (20 x 30 microns) and/or typical lesions in the midgut upon necropsy 5-6 days PI were present in 42% of the samples. Eimeria tenella was suspected in 24 of 43 samples on the basis of oocyst size but was confirmed in six samples (14%) at necropsy of infected birds with typical lesions in the ceca (hemorrhage, coagulated blood in the lumen, and thickened cecal mucosa). Characteristic lesions of Eimeria acervulina and ovoid oocysts (14 x 18 microns) were present in all 40 positive samples (93%). Lesions in the lower small intestine of birds inoculated with two of the samples (5%) were typical of Eimeria brunetti. These results confirmed the presence of six species of Coccidia in Argentine poultry (all except Eimeria necatrix, which is known from previous reports) and demonstrated a prevalence and intensity for E. praecox and E. mitis that were previously unreported.