Congenital transmission of T. spiralis infectio in BALB/c mice was studied. Pregnant mice were each infected with 300 larvae 5, 7, 15 and 17 days after fertilization. Newborn mice were examined by artificial digestion of muscles. Out of 6 offspring born to the mother-mouse infected 7 days after fertilization, two offspring were found to be infected, 7 and 24 larvae were recovered respectively. Other 7 female mice were first infected with T. spiralis larvae and then gestated, only the offspring born to the mother-mice fertilized 8 and 22 days after infection were found to be infected with a larval burden ranging from 1–3 larvae per animal. All of the larvae recovered from the offspring were the non-encapsulated larvae. The cross-fostering in which one-day old young born to healthy mother-mice were nursed by infected mothers for 21 days, showed that no young were found to be infected. These findings showed that tansplacental transmission of T. spiralis could occur in mice, if the female were infected during mid-pregnancy or fertilized in 1 month after infection (e.g., infected in one month before fertilization). The larvae transmitted from maternal-to-neonatal mice may be migrating. Transmammary transmission of T. spiralis was not observed.