A complete ground and pelleted feed was compared to free choice feeding of whole wheat and a pelleted protein concentrate during three experimental infections with coccidia in broiler chickens. At 22 d of age birds were inoculated with different doses of sporulated oocysts of a cecal species (Eimeria tenella) in experiment 1 or intestinal species E. maxima or E. acervulina in experiments 2 and 3, respectively. The effects of diets were assessed on weight gain, hematocrit (during cecal coccidiosis), serum coloration (during intestinal coccidiosis), oocyst excretion, and lesion score until 7 d post-inoculation. In experiment 1 before inoculation, the birds fed whole grain had more beneficial microflora with lower counts of coliform bacteria. As shown by oocyst output and lesion score, whole wheat feeding increased parasite development during infection with the cecal parasite E. tenella. This led to significantly lower weight gain with whole wheat than with ground wheat from 5 to 6 d post-inoculation and to lower hematocrit at the highest infective dose. Parasite development in experiments 2 and 3 was similar among diets, during intestinal infection with E. maxima and E. acervulina, respectively, with no significant differences in lesion score. During the acute phase (4 to 7 and 3 to 5 d post-inoculation), when a difference appeared between diets, whole wheat fed-birds were always more affected than ground diet-fed birds in terms of serum coloration and weight gain. These results might be explained by modifications of digestive physiology and intestinal microflora by whole wheat.