Eight floor pen trials were conducted to determine the effect of arprinocid on the weight, feed efficiency, and histological lesions of broiler chickens exposed to coccidiosis. In three trials, the levels of arprinocid were 0, 40, 60, 70, 80, and 90 ppm, and in the other five trials, 0, 50, 75, and 100 ppm. Histological lesions were significantly (P less than .05) reduced in the medicated birds. Weights were not significantly (P less than .05) different between birds given 40, 60, or 70 ppm in the first series of trials no 50 and 75 ppm drug in the second and third series, although in both series the treated birds were significantly (P less than .05) heavier than the control birds. Feed efficiency was improved by all levels of arprinocid in the first and third series of trials and by 75 ppm in the second series.