Because of uniformly poor results from attempts to infect rabbits with S. mansoni by intraperit. injn. of cercariae, a study was undertaken of the routes of migration and stages of development in various tissues and portions of the circulation of rabbits and hamsters following intraperit. and percutan. infections. In rabbits infected intraperit. few worms were able to reach the lungs and portal circulation, suggesting high mortality in the peritoneal cavity or adjacent tissues. In rabbits following percutan. infection, and in hamsters following either method of exposure, a considerable proportion of the worms reached the lungs and portal circulation. The development of worms which reached these regions in rabbits infected intraperit. was slower than in rabbits infected percutan. suggesting that some of the worms from the intraperit. infection had suffered damage not apparent in worms from percutan. infections. Development to maturity was slower in rabbits infected percutan. than in hamsters similarly infected, indicating that the rabbit is a less favorable host. The nature of the condition in the peritoneal cavity of rabbits which is unfavorable for the cercariae of S. mansoni, and its possible immunological significance remain to be determined.