In previous studies true-breeding stocks of Biomphalaria glabrata were derived testing 100% or 0% susceptible as juveniles to a Puerto Rican strain of Schistosoma mansoni. When adults of these genetic stocks of B. glabrata were exposed to miracidia of the same strain of S. mansoni, snails of some of the juvenile-susceptible stocks were found to have become refractory at maturity. True-breeding snail stocks of three susceptibility types were derived by selection and self-fertilization through several generations: juvenile and adult refractory, juvenile and adult susceptible, and juvenile-susceptible/adult-refractory. Crosses demonstrated that the adult insusceptibility observed in some of the juvenile-susceptible stocks was determined by a single dominant gene, with the character showing simple Mendelian inheritance. Snails of a fourth susceptibility type showed juvenile susceptibility but modification of adult susceptibility, with unpredictable infection frequencies and delayed parasite development. Clonal stocks of the various susceptibility types, with their lineage recorded in diagrams, are identified for use in other experimental studies.