The effect of diclazuril medication on the development of natural immunity to Eimeria tenella was determined. Birds in two treatments, nonexposed, non-challenged (NENC) and nonexposed, challenged (NEC), were from a holding group raised under conditions designed to prevent coccidial infection. Birds in five other treatments, from the same hatch and source as birds in the first two treatments, were from a floor pen study involving a natural exposure to Eimeria spp. These birds were assigned to the current study based on their respective treatments in the original floor pen study as follows: unmedicated (UNM), 66 ppm salinomycin (SAL) + 50 ppm roxarsone in the starter and grower diets to Day 28 (SAL 28), 66 ppm SAL + 50 ppm roxarsone in the starter diet and 1 ppm diclazuril (DIC) in the grower diet to Day 28 (DIC 28) or to Day 35 (DIC 35), or in the grower and finisher diets to Day 42 (DIC 42). Each treatment comprised three floor pens of 10 birds (female) in a randomized complete block design. All birds were fed an UNM finisher diet during the 7-d challenge study. Birds in each treatment, except NENC, were individually inoculated (p.o.) with 1 x 10(5) Eimeria tenella sporulated oocysts on Day 1. Based on bird performance and cecal lesion scores, birds in the DIC 35 and 42 treatments had a low immunity to the challenge infection, birds in the DIC 28 treatment were partially immunized, and the highest levels of immunity were observed in the UNM and SAL 28 treatments.