ABSTRACT
In this study ,upon exposure of laboratory Egyptian B. alexandrina and B. glabrata snails to its homologus strain of S. mansoni, two different phenotype strains " susceptible and resistant " were distinguished . The aim of this study is to investigate the biological differences between the two types of Biomphalaria snails either susceptible or resistant at different ages. The investigated parameters included mortality, survival, percentage of susceptibility / resistance to parasitic infection and fecundity. The results of the preliminary exposure experiment revealed that parental snail stock was not completely susceptible and tend to be partially refractory to infection. The results also showed a strong association between the levels of egg production and susceptibility to parasite infection. Where breeding experiment revealed that, total egg number, E/M and EM values produced by resistant phenotype of both snail species were more than those produced by susceptible progeny snails. Also, it was noticed that, egg production is associated inversely proportion with snail age