Several groups of mice were infected with graded doses of Schistosoma japonicum. The 50 per cent lethal dose under the experimental conditions was 58 cercariae. The total number and the sex distribution of developing worms were important factors in bringing about death. In very heavy infections death was due primarily to thrombosis resulting from large masses of worms and, in relatively lighter infections, to extensive oviposition. Study of over 15,000 serial sections from infected mice sacrificed at regular intervals indicates that most of the developing worms migrate from lungs to liver through blood vessels. However, only a portion of schistosomula found in the liver were in the portal veins. Large numbers of young worms were observed in central veins and parenchyma.