Marine algae are economically important aquatic photosynthetic organisms act as a source of highly bioactive metabolites that might be useful against predators and parasites. In the present study, aqueous and ethanol extracts of four algal species of marine algae; Pterocladia capillacaea (P. capillacaea), Jania rubens (J. rubens), Ulva lactuca (U. lactuca),and Enteromorpha intestinalis (E. intestinalis) were tested for their lethal effect on Lymnaea natalensis (L. natalensis) snail, the intermediate host of the trematode parasite; Fasciola gigantica, (F. gigantica). Preliminary experiments were done to determine the molluscicidal toxicity of different concentrations of aqueous and ethanol algal extracts against the studied snail. Five concentrations of aqueous and ethanol extracts; 100, 200, 300, 400 and 500 ppm for each algal species were used. LC₅₀ for each extract was calculated daily until the end of the experiment. It is clear from the results that molluscicidal effect of algal extracts is concentration-dependent. The most potent extracts as proved by calculated LC₅₀ are  P. capillacaea and J. rubens aqueous extract (red algae); 111.3 and 127 ppm respectively and U. lactuca ethanolic extract (green algae) 121 ppm. The current study introduces marine algae of promising molluscicidal activity on L. natalensis snail. P. capillacaea and J. rubens aqueous extracts and U. lactuca ethanolic extract are the most potent and highly significant ones in the net reproductive rate and reduction. Further investigations for isolation of active principals of the algal potent extracts for evaluation as molluscicides and oranthelmintics are necessary required.