Prior infections with trichina (125 larvae per mouse) did not reduce the numbers of schistosomes developing from exposures of 50 cercariae per mouse. Differences in the numbers of adult schistosomes recovered from mice with trichinosis and from controls were not significant (1[plus or minus]2 worms in female mice and 4[plus or minus]2 worms in male mice). However, prior infections with trichina did reduce the numbers of schistosomes developing from exposures to 200 cercariae per mouse. Differences in the numbers of adult schistosomes recovered from mice with trichinosis and from controls were significant (25[plus or minus]6 worms).