Activity of the following glycosidases was detected in the plasma of the freshwater snail Biomphalaria glabrata: β-D- fucosidase, β-D-glucosidase, β-D-galactosidase, β-D-mannosidase, β-D-glucuronidase, N-acetyl-β-D-galactosaminidase, N-acetyl-β-D-glucosaminidase, and lysozyme. At the physiological pH (7·2–7·4) of snail haemolymph, enzymatic activity was about 10–50% of the maximum activity at each enzyme's respective acid pH-optimum. Schistosome-susceptible B. glabrata showed lower plasma protein concentration and significantly lower enzymatic activities (U/mg protein) than schistosome-resistant snails. Changes in glycosidase activity levels correlate with the progress of infection. After successful schistosome invasion, activities of plasma glycosidases but not the concentration of total plasma proteins increased significantly during the first 2 days in both snail strains. Thus, most tegumental glycoproteins of schistosome larvae can be altered by humoral host glycosidases. The detection of only very low activities of hexosaminidases leads to the hypothesis that GalNAc/GlcNAc may be involved in the process of non-self recognition. At 4 days post-infection, glycosidase activities were identical or slightly below the levels found in naive snails. At this time of infection the parasite is encapsulated and destroyed by haemocytes of resistant snails. In susceptible snails, however, the schistosomes have transformed into sporocysts and will complete their life-cycle without eliciting effective defence reactions. After >30 days post-infection, when cercariae are fully developed in susceptible snails, plasma protein concentration decreased significantly, whereas glycosidase activities were elevated.