Diclazuril, a new anticoccidial drug, was tested in poultry against six Eimeria species either in single species infections in dose titration studies or in mixed species infections in floor pen trials. The dose titration studies in cockerels indicated that diclazuril at dosages of 10, 5, 1, and .5 ppm was highly active against all major pathogenic species: E. tenella, E. acervulina, E. necatrix, E. brunetti, E. maxima, and E. mitis and effective in terms of weight gain and suppression of mortality, dropping scores, and oocyst counts. In two floor pen trials diclazuril was fed for 6 wk to broiler chickens experimentally infected either with E. tenella and E. acervulina or with E. necatrix and E. brunetti. Dose levels of 10, 5, and 1 ppm suppressed mortality and lesion scores. Even at 1 ppm the mean terminal body weight, feed conversion, and productivity index of treated birds were comparable to results for the uninfected, unmediated controls. In these pilot studies, involving 1,020 Hisex and 1,000 Hubbard broiler chickens, it has been demonstrated that diclazuril at the dose level of 1 ppm in the diet is an excellent anticoccidial without any adverse effects.