Biomphalaria alexandrina snails are members of the phylum Mollusca and are known to have an effective innate defense system consisting of cellular and humoral defense factors which can be modulated by immunostimulation. The effect of sodium alginates on B. alexandrina snails and their hemocytes, antioxidant enzymes and infection dynamics was assessed under laboratory conditions. The influence of sodium alginates on the infection rate of snails with Schistosoma mansoni miracidia under simulated natural conditions was evaluated. Exposing snails to sodium alginates resulted in a considerable increase in the overall number of hemocytes and the most significant increase was 6650 hemocyte/ml after 7 days of exposure to 0.5 mg/ml sodium alginates, compared to 1950 hemocyte/ml in control snails. Superoxide dismutase, catalase, and glutathione activities were reduced. Lipid peroxide levels were significantly elevated after sodium alginates' exposure. Exposure of snails to sodium alginates under simulated natural conditions reduced the infection rate of snails by 28.3%, 18.5% and 55.4% after treatment with 0.1 mg/ml, 0.5 mg/ml and 1.0 mg/ml respectively. In general, exposure to sodium alginates showed a pattern of delayed infection, as well as sporocysts distortion and degeneration when compared to normally infected snails.  It is concluded that sodium alginate is a promising candidate as a biocontrol agent for schistosomiasis in Egypt as it enhanced B. alexandrina snails' immune ability to overcome infection with S. mansoni miracidia.