To clarify the character of the anticoccidial activity of diclazuril a series of battery trials was conducted. Diclazuril showed excellent anticoccidial activity in the infection of chickens with Eimeria tenella, E. necatrix or E. acervulina at the feeding level of 0.1 ppm. When diclazuril was administered in combination with a nucleic acid precursor, uracil, uridine, orotate or orotidine, the reduction of the activity of diclazuril to the infections induced by above species was not observed. While, bloody droppings with severe cecal lesions were resulted, when diclazuril was administered in combination with uridine 5(1)-diphosphoglucose (UDPG) or its N-acetyl amine (UDPGNAC) to chickens infected with E. tenella. While, body weight gain of the birds and oocyst output was not affected by these combination-treatment. Results demonstrated that the antagonistic effect of UDPG and UDPGNAC to diclazuril was partial. The possibility of the cross resistance between diclazuril and 6-azauracil (AzU) in E. tenella was investigated using two populations induced resistance to AzU or diclazuril. The results demonstrated that the cross resistance does not exist between AzU and diclazuril, indicating that the mode of action of each drug is different.