Anticoccidial effect of diclazuril was assayed in broilers under field conditions. Diclazuril was mixed in broiler's feed at concentrations of 0.125, 0.25, 0.5 and 1.0ppm. Salinomycin was also used with its concentration of 50ppm in feed and served as the control. These five medicated feeds were provided to 5 groups of chickens, respectively, throughout their rearing term until one week before they were sent out for slaughter. Efficacy of the drugs was assessed by their suppressing effects on oocyst production in medicated chickens. Unmedicated control chickens suffered from coccidiosis with clinical sign of bloody droppings. Considerably large numbers of oocysts were observed in feces of control chickens throughout all examining times. Salinomycin-medicated chickens also showed continuous oocyst excretion with certain numbers of oocysts starting when the chickens were 14 days old. In contrast, oocyst excretion was completely inhibited by the medication of diclazuril at the levels of 0.25 ppm or more in feed, while only countable amounts of oocysts were permitted at 0.125 ppm in feed. When percent of sporulation was estimated for oocysts derived from 0.125ppm-dicrazuril medicated chickens, it was substantially lower suggesting that diclazuril has an inhibiting effect not only in oocyst productions but in oocyst sporulation. In the laboratory test, anticoccidial effect of diclazuril was confirmed with results similar to those of the field trial.