Biomphalaria alexandrina snails were examined for their tolerance to cadmium (Cd) toxicity through three generations produced by self-fertilization by exposing snails to serial concentrations of Cd and selection for the most tolerant ones through the successive generations. Results showed a gradual increase in the LC50 & LC90 values via successive generations indicating tolerance upgrading phenotype. Also, the selected and non-selected snails were examined for susceptibility to Schistosoma mansoni and hematological criteria. B. alexandrina showed refractory tendency with significant increase in hemocytes; granulocytes; indicating that defense mechanism was stronger than other ones. The dendro-gram based on the similarity matrices of ISSR-PCR banding patterns resulted with ten random primers, showed that the selected groups of the 1st and 2nd generations were the most similar and clustered together forming a distinct group that in turn cluster with nonselected laboratory group that in turn cluster with parent group (field, non-selected), while the 3rd selected generation was the most divergent group, indicating its genetic drift. The mating system
and selection succeeded in evolution of Cd tolerance snail isolate; characterized genetically, physiologically, showed refractory tendency to S. mansoni infection.