Abstract: Objective To clarify the relationship between some cytokines and gametocyte infectivity of Plasmodium yoelii to the mosquito vector.Methods We measured oocyst formation in mosquitoes,levles of IFN-γ and TNF-α in ser a from infected mice and mosquito midguts as well as early development of sexual stage parasites in the mosquitoes 8h after blood meal.Results On the third day after infection,the serum level of IFN-γ was significantly higher than that in other groups.However,there were no differences in the serum levels of TNF-α between normal and infected mice.At the same time oocyst formation exhibited a peak in mosquito midguts.The cytokine levels and ookinete formation inthe mosquitoes were similar to those above.Conclusion IFNγ is not direct effector molecules inhibiting gametocyte infectivity in the course of Plasmodium yoelii transmission-blocking during the early phase of infection.