Aiming to further characterize the haemocyte subsets in Biomphalaria snails, we have performed a detailed flow cytometric analysis of whole haemolymph cellular components using a multiparametric dual colour labelling procedure. Ethidium bromide/acridine orange fluorescence features were used to first select viable haemocytes followed by flow cytometric morphometric analysis based on the laser scatter properties (forward scatter-FSC and side scatter-SSC). Our findings demonstrated that B. glabrata (BG-BH, highly susceptible to S. mansoni) and 2 strains of B. tenagophila (BT-CF, moderately susceptible and BT-Taim, resistant to S. mansoni) have 3 major circulating haemocyte subsets, referred to as small, medium and large haemocytes. The frequency of small haemocytes was higher in BG-BH, while medium haemocytes were the most abundant cell-type in both B. tenagophila strains. Schistosoma mansoni infection resulted in early reduction of large and medium circulating haemocytes followed by an increase of small haemocytes. Although parasite infection induced haemocyte alterations in all Biomphalaria strains, the response was particularly intense in BT-Taim, the parasite-resistant snail. Interestingly, the trematode infection induces changes in haemocytes with less granular rather than in those with more granular profile. The results indicated that, in B. tenagophila of Taim strain, circulating haemocytes, especially the medium and high subset with less granular profile, are very reactive cells upon S. mansoni infection, suggesting that this cell subset would participate in the early parasite destruction observed in this snail strain.