The growth and development of Gymnophalloides seoi were studied in C3H/HeN mice and effects of immunosuppression of the host on the worm development were observed. Two hundred metacercariae of G. seoi were orally administered to each mouse, and worms were recovered on days 1, 3, 5, 7, 14 and 21 post-infection (PI). The worm recovery rate was significantly higher in immunosuppressed (ImSP) mice than in immunocompetent (ImCT) mice except on days 1 and 3 PI. The worms attained sexual maturity by day 3 PI with eggs in the uterus, and worm dimensions and the number of uterine eggs continuously increased until day 14 PI in ImSP mice. Worms recovered from ImSP mice were significantly larger in size than those from ImCT mice on days 1 and 3 PI, and the number of uterine eggs was significantly larger in ImSP mice on days 5 and 7 PI. Genital organs such as the ovary, testes, and vitellaria, that were already developed in the metacercarial stage. grew a little in size until day 14 PI. The results show that the C3H/HeN mouse is, though not excellent, a suitable laboratory host for G. seoi.