The effect of riboflavin supplement in Eimeria tenella-infected chickens was evaluated. Mortality, fecal consistency, and oocysts per gram of feces were monitored for groups of E. tenella-infected chickens administered a basal diet supplemented with either riboflavin, the anti-coccidial drug amprolium, or with both compounds. The number of oocysts shed per chicken in the riboflavin-treated group was significantly higher than the positive non-treated control group. No significant difference in oocyst number between the amprolium-treated group and riboflavin plus amprolium-treated group was observed. Thus, the addition of 0.8 g/kg of riboflavin to basal diet can increase oocyst number in E. tenella-infected chicken, but has no effect on the efficacy of amprolium.