Scattering and backscattering properties were measured for 4 main fresh water alga species cultured in the laboratory: Microcystis aeruginosa, Chlorella vulgaris, Cyclotella meneghiniana and Cryptomonas ovata. Chlorophyll a-specific scattering and backscattering were used to represent scattering ability. M. aeruginosa had the strongest scattering and backscattering ability, followed by C. meneghiniana, and C. vulgaris had the weakest. M. aeruginosa and C. meneghiniana showed greater backscattering ability than C. vulgaris and C. ovata. The main factor affecting backscattering was the ratio of chlorophyll a to phycocyanin content. Backscattering coefficient varied with chlorophyll a content and chlorophyll a proportion to total pigment. We found a good correlation between cell size and backscattering coefficient.