Abstract: Coccidiosis and its following infection with clostridia in chicken are two common diseases in chicken. Ampicillin is a highly recommended therapy in clostridia infections in chicken. However, the effect of coccidiosis on ampicillin pharmacokinetics is not known. In this study, chicken infected with Eimeria tenella showed significant changes in ampicillin bioavailability and pharmacokinetics. Compared with noninfected chicken, intravenous injection of ampicillin in the infected chicken showed higher distribution and elimination constants and statistically significant higher AUC and MRT. Furthermore, oral administration in the infected chicken produced higher mean absorption time, delayed tmax, lower Cmax, smaller AUC value and lower bioavailability. Coccidiosis is the major predisposing factor for clostridia infections. Therefore, the use of ampicillin is highly expected following coccidiosis. Based on these results, monitoring and adjustment of ampicillin dosing could be practiced during the presence of coccidiosis.