Bulinus beccari, intermediate host of Schistsoma haematobium, and Biomphalaria pfeifferi, intermediate host of S. mansoni have different geographical distribution in Jazan Region. The role of rain fall, ionic composition of water. water temperature and pH Grades were studied. Fluctuation of snail population densities were accompanied with corresponding fluctuation rates of rainfall. B. pfeifferi showed a wider and heavier shell than B. beccari, which might enable it to resist the effects of flash floods. B. beccari showed a higher tolerance of total dissolved solids and calcium carbonate than B. pfeifferi. Average maximum tolerated concentration of dissolved solids and calcium carbonate for B. beccari was 1254ppm and 813ppm, while that of B. pfeifferi was 455ppm and 603ppm. Average water temperature of B. beccari habitats was 25-36.3 degrees C while that of B. pfeifferi was 25-28 degrees C. No significant statistical difference in different pH grades was observed.