Background: The internal defense system of snails consists of both cellular and humoral components. Circulating haemocytes are the principle line of cellular defense. The susceptibility of fresh water snails of genus Biomphalaria to infection by Schistosoma mansoni (S. mansoni)is linked to the haemocytes present in the haemolymph. While the humoral component includes lysosomal enzymes, that helps in recognition of pathogens and parasites.  Material and Methods: In the present work, Biomphalaria alexandrina (B. alexandrina),snails were exposed individually to S. mansoni infection; according to their response they were classified into susceptible group (shed cercariae) and resistant group (failed to shed cercariae). Snails not exposed to infection were considered as control group. Flow cytometric analysis was carried out for detecting apoptosis of B. alexandrina haemocytes isolated from pooled haemolymph and tissue.      Results: This study is used to discriminate and count the percentage of viable, apoptotic, necrotic, and dead cells in haemolymph of all studied groups. Comparing to control group, we observed that viable cells were the most common cells that significantly decreased in both infected and non-infected groups (P<0.001, P<0.01 respectively), and showed significant decrease in infected in comparison with non-infected group (P<0.01).  The haemolymph in infected group showed that dead cells were the most common cells (more than the viable, apoptotic and necrotic cells). At the same time, the apoptotic cells showed decrease in the condensation and de- condensation of DNA. This means failure to restore DNA. Conclusions: There was a significant variation in the number of the cells between susceptible and resistant snails. In susceptible snailsthe defense process failed, this was confirmed with increase of the dead cells in the infected haemolymph. While in resistant snails the viable spreading cells described as immunological active, are predominantly found in the haemolymph and tissue.