A floor pen trial was conducted to evaluate the efficacy of the fermentation product lasalocid [11054-70-9] against coccidiosis of chickens. Lasalocid was included in the feed at 4 levels: 75, 90, 100, and 125 ppm. The lasalocid medicated and unmedicated control chickens were inoculated individually with a total of 195,000 sporulated oocysts of Eimeria species consisting of E. tenella, E. necatrix, E. acervulina, E. maxima, and E. brunetti. All 4 levels of lasalocid were effective in preventing coccidiosis among infected groups, as compared to the unmedicated infected controls, where severe morbidity and mortality had occurred. Lasalocid levels of 75, 90, and 100 ppm significantly reduced the intestinal lesions as compared to the unmedicated controls, and 125 ppm lasalocid completely prevented intestinal lesions due to coccidiosis. The average weight gain and feed conversion was superior on lasalocid treatment of 75, 90, and 100 ppm as compared to the treatment levels of 125 ppm and the appropriate controls. Statistically, results of lasalocid treated chickens were significantly better than infected unmedicated chickens.