Schistosomiasis ranks the second amongst parasitic diseases affecting human. To reduce the incidence of this disease, several strategies were applied including snail control. Chemical control methods have many drawbacks, hence biological control became the concern of many studies. In this study, eight algal species were tested against Biomphalaria alexandrina snails; the intermediate host of Schistosoma mansoni. The bioassay tests revealed that Cystosiera barbata and Dictyota dichotoma were the most effective species, where the LC₅₀ values were 2200 ppm and 560 ppm consequently. Prolonged exposure to LC₂₅ (280 ppm) of D. dichotoma resulted in death of snails after two weeks. In addition, alterations in digestive glands of treated snails with LC₅ (220 ppm) and LC₂₅ (280 ppm) of C. barbata and D. dichotoma, respectively were obvious, as vacuolation of digestive cells were recorded after exposure of snails to LC₅ of C. barbata. Moreover, exposure of snails to LC₂₅ of D. dichotoma resulted in noticeable vacuolation of digestive cells and degenerated secretory cells; the lumen was nearly disappeared. The maximum elevations in the activities of both aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were observed after 2 weeks of exposure to LC₅ (56 ppm) and LC₁₅ (660.7 ppm) of D. dichotoma and C. barbata, respectively. The total protein contents of treated B. alexandrina snails were significantly different from that of control group after 2 weeks of exposure to different concentrations of both effective algae. Testing the sublethal concentrations of both effective algae on the Daphnia pulex showed that LC₅ (220 ppm) of C. barbata and LC₁₅ (168 ppm), LC₂₅ (280 ppm) of D. dichotoma were the least harmful to these crustaceans.