Abstract: Objective: To investigate the influence of maternal infection by staphylococcal enterotoxin B(SEB) during pregnancy on the development of fetal thymus. Methods: The pregnant rats at the gestational day(GD) 16 were randomly divided into two groups:the experimental group of 12 rats injected intravenously by 15 μg SEB and the control group of 12 rats by vehicle. At GD 21 the fetuses of two groups were acquired after the pregnant rats were killed,then the weights of placenta,fetal thymus and the whole fetal body were measured. The percentages of CD4⁺ and CD8⁺ T cell subsets in thymus was detected by flow cytometry. Results: Maternal infection by SEB during pregnancy decreased the weights of fetal thymus as well as the percentages of both CD4^-CD8⁺ and CD4⁺ CD8⁺ T cells, while the percentage of CD4⁺ CD8^-T cells was increased compared to that of the control group(P < 0.01 and P < 0.05). But the weights of placenta and the whole fetal had no difference between the two groups(P > 0.05). Conclusions: The thymus development of fetus can be affected by maternal infection by SEB during pregnancy.