Susceptibility to infection with Schistosoma mansoni was studied in juvenile Biomphalaria glabrata by isolated selfing and by crosses. True-breeding snail stocks were derived testing 100% or 0% juvenile susceptible. Crosses suggested that juvenile susceptibility was regulated by a complex of four or more genetic factors. Genes for insusceptibility could be transmitted by susceptible snails and genes for susceptibility by refractory snails.