Abstract: The pathogenicity and development of transgenic E. tenella strain TE1, which could express yellow fluorescent protein (YFP) and dihydrofolate reductase thymidylate synthase derived from Toxoplasma gondii (TgDHFRTS), in chickens were compared with that of the parental strain BJ to determine the effect of foreign gene on transgenic parasite. Results indicated that the fecundity of transgenic parasite (TE1) reduced at least 4 times than that of BJ strain. Low dosage of TE1 strain induced less pathogenic to chickens than BJ strain, but chickens inoculated with high dosage of TE1 oocysts displayed severe pathogenicity and certain mortality as well as BJ strain. In addition, trophozoites, the first generation and the second generation meronts and merozoites, microgamonts and macrogamonts of transgenic parasite TE1 were seen under flurescent microscope as that of BJ strain. More interestingly, not all four sporonts in the sporulating transgenic oocysts express YFP. These findings suggest that expressions of YFP and TgDHFRTS genes reduced pathogenicity and reproductive potential of transgenic E. tenella to some extent. Recombination between homologous or nonhomologous chromosomes appears during zygotic meiosis in E. tenella.