Sporozoite of Eimeria tenella inoculated into the allantoic cavities of embryonating eggs completed their life-cycle in the chorioallantoic membranes (CAM ) and produced viable oocysts. And the strain continued to adapt to the CAM through the period of the passages. In embryos, the reproduction of the strain, judged by oocyst production increased, but the pathogenicity, judged by mortality of embryo decreased, with increasing numbers of passage in eggs.