Abstract: A line of Eimeria mitis with an abbreviated life cycle (i.e. a “precocious ” line) was derived from the Zhuozhou strain by repeated passage of the oocysts which were the first to be recovered from infections in chickens. The prepatent period was reduced by 19 hours. The precocious parasite is less pathogenic than the parent strain, has a reduced reproductive potential, and remains much of the immunogenicity of the parent strain. Shiqiza chicks inoculated with the parasites of the precocious line were well protected against challenge with the Zhuozhou strain. Through a relaxed selection of 10 passages, it was showed that the precocious line is genetically stable in its prepatent time, reproduction peak and potential, and pathogenicity.