CD4~+CD25~+T cells play a major role in modulating immune response,but few reports havebeen published about schistosomiasis.Here,we investigated the changes in CD4~+CD25~+T cell populations inspleens and mesenteric lymph nodes of mice infected with Schistosoma japonicum.The proportions ofCD4~+CD25~+T cells in total CD4~+T cells were analyzed by flow cytometry.CD25 and Foxp3 expression wasmeasured by real-time quantitative polymerase chain reaction.The suppressive activities of CD4~+CD25~+Tcells were detected by in vitro proliferation of splenocytes.Evidence showed that the percentage of CD4~+CD25~+T cells was the same as controls 3 weeks post-infection.At the acute stage of infection,the percentagedecreased significantly.However,at the chronic stage of infection,it rebounded to normal levels or evenhigher.The expression of the CD25 and Foxp3 showed gradual increase along with the infection progress.Invitro experiment also showed the strong suppressive effect of CD4~+CD25~+T cells,isolated during the chronicstage,on proliferation of the CD25~-splenocytes.This is the first time that the dynamics of CD4~+CD25~+T cellpopulations was demonstrated in mice infected with schistosomiasis.In conclusion,our data indicated thatCD4~+CD25~+cells might be involved in the immune modulation during S.japonicum infection,which en-hances current knowledge of the mechanisms of the immuno-downregulation and re-infection inschistosomiasis. CD4 ~ + CD25 ~ + T cells play a major role in modulating immune response, but few reports havebeen published about schistosomiasis. Here, we investigated the changes in CD4 ~ + CD25 ~ + T cell populations inspleens and mesenteric lymph nodes of mice infected with Schistosoma japonicum. These proportions of CD4 ~ + CD25 ~ + T cells in total CD4 ~ + T cells were analyzed by flow cytometry. CD25 and Foxp3 expression wasmeasured by real-time quantitative polymerase chain reaction. The suppressive activities of CD4 ~ + CD25 ~ Tcells were detected by in vitro proliferation of splenocytes. Evidence showed that the percentage of CD4 ~ + CD25 ~ + T cells was the same as as 3 weeks post-infection. At the acute stage of infection, the percentage greatly increased. Host, at the chronic stage of infection, it rebounded to normal levels or evenhigher. The expression of the CD25 and Foxp3 showed gradual increase along with the infection progress. Invitro experiment also showed the strong suppressive effect of CD4 ~ + CD25 ~ + T cells, isola ted during the chronic stage, on proliferation of the CD25 ~ -plenocytes. This is the first time that the dynamics of CD4 ~ + CD25 ~ + T cellpopulations was demonstrated in mice infected with schistosomiasis. In conclusion, our data indicates that CD4 ~ + CD25 ~ + cells might be involved in the immune modulation during S. japonicum infection, which en-hances current knowledge of the mechanisms of the immuno-downregulation and re-infection inschistosomiasis.