Biomphalaria tenagophila "hybrids" originating from the crossing of albino strains from Belo Horizonte (BH) or from Joinvile (SC with melanic specimens from Cabo Frio (CF), Taim (Ta) or from Curitiba (PR), submitted to infection with Schistosoma mansoni (LE strain, Belo Horizonte, MG), presented the following results: in F1, TaSC, PRSC and CFTa "hybrids" showed susceptibility rates of 4.5%, 12.5%, and 11.2%, respectively; in F2, all "hybrids" were shown negative, and in F3, only one albino specimen (CFBH)². TaBH offspring was infected with LE strain. As far as controls were concerned, B. glabrata showed susceptibility rates of 66.7% and 93.6% to LE strain; B. tenagophila from Joinvile presented 17.1% and 33.3% to SJ strain, and BHTa and BHCF hybrids showed 6.0% to 53.8% to SJ strain too. It was noted a great influence of mother strain on susceptibility rates. Since the offsprings originating from crossing of unsusceptible strains to LE (CF, Ta and BH) were infected, the use of hybrids to defect susceptibility gens in such strains is discussed. Further, the necessity of the use of a greater number of miracidia in infection tests, and the lack of correlation between the frequency of host-parasite contacts and infectivity rates are discussed. Considering the fact that those infectivity rates are dependent on genetic characteristics, previously existing in the population, and not on the frequency of contacts, LE strain could be a genetic variety (or race) different from SJ strain, endowed with little ability to infect different populations of B. tenagophila, except for that from Joinvile (SC).