Previous studies revealed an ameliorating effect of Eimeria praecox on concurrent E. maxima infection, such that weight gain, feed conversion ratio, and intestinal lesions were nearly identical to uninfected or E. praecox-infected controls. The purpose of the present study was to determine if protective immunity against E. maxima challenge infection developed in chickens infected with both E. praecox and E. maxima. Day-old chickens were infected with 10(3)E. praecox, 10(3)E. maxima, or a mixture of 10(3)E. praecox and 10(3)E. maxima oocysts. Chickens were then challenged at 4 weeks of age with 5x10(4)E. praecox or 5x10(3)E. maxima oocysts and clinical signs of coccidiosis were assessed 7 days post-challenge. Relative to non-challenged controls, naïve chickens or chickens immunized with E. praecox displayed a 32-34% weight gain depression after challenge with 5x10(3)E. maxima oocysts. In contrast, chickens immunized with either E. maxima oocysts alone or a combination of E. praecox and E. maxima oocysts displayed complete protection against lower weight gain associated with E. maxima challenge. Also, protection against decreased feed conversion ratio and intestinal lesions was observed in single E. maxima- or dual E. maxima+E. praecox-immunized chickens. These findings indicate that co-infection of chickens with E. maxima and E. praecox does not prevent development of immunity against E. maxima or E. praecox challenge.