Experiments were conducted to determine whether chukar partridge (Alectoris chukar) chicks would develop protective immunity after inoculation with coccidia. Young chukar chicks in battery cages inoculated with 100 or more oocysts of Eimeria kofoidi or Eimeria legionensis had significant protection at challenge 4 wk later, as measured by greatly reduced oocyst shedding and improved weight gain as compared with unvaccinated, challenged controls. However, when birds were housed in litter pens and vaccinated by various regimens (including two species of chukar coccidia at 100/dose), coccidiosis rapidly spread through all treatments and caused significant mortality. Vaccination with Coccivac-T or with 100 oocysts of Eimeria dispersa did not prevent mortality resulting from accidental contamination, and feed treatment with a Lactobacillus competitive-exclusion product had no benefit. Most if not all of the mortality was from E. kofoidi. This study illustrated the natural fecundity of chukar coccidia in a floor-pen environment where multiplication rate and reinfection combine to produce clinical disease from a small original exposure. Further, these results cast doubt on the potential use of low doses of live oocysts as a vaccine in the chukar partridge.