Abstract
Six attenuated lines of Eimeria species of the chicken of USA origin were produced by selection for precociousness. These lines were less pathogenic, judged by body weight changes and intestinal lesions in infected chickens, than the parent strains from which they were derived. The immunogenicity of the precocious lines and their parent strains was similar. The use of these attenuated Eimeria Unes for immunological control of coccidiosis is discussed.