After 6 to 8 months of storage, cultures of sporulated Eimeria acervulina and E. tenella oocysts ahd a marked drop in the number of sporocysts that survived grinding, sporozoites that survived after excystation, and sporozoites that penetrated chick kidney cells in vitro. The rate of excystation was unaffected by storage of up to one year. In vivo pathogenicity, based on weight gain, lesion score, and plasma pigment, declined after 5 months of oocyst storage. The reduction in pathogenicity in vivo could be compensated for by adjusting the inoculation dosage to reflect the loss of infectivity seen in the in vitro test.