Abstract：One precocious line was derived from the Shanxi strain of Eimeria tenella．The reproductivity and pathogenicity of the precocious line and parental strain were examined．The results showed that the prepatent period of the precocious line was 21 hours which is shorter than that of the parental train．the reproductive potential of precocious line was 40% of the parental strain．When chickens were infected with same dosage of precocious line oocysts and the parental strain oocytes，The gained weight of chicken infected with precocious line was higher than that with the parental strain，and the pathological changes scores of the chicken infected precocious line was less than that of the parental strain．The LD50 of parental strain and precocious strain were 7.52×104、27.64×104 to 11-day chicken respectively. The pathogenicity of the precocious line was attenuated significantly.