<p>In this study, the effects of the different densities of Chlorella vulgaris (0.05 x 106 , 0.1 x 106 , 0.2 x 106 , 0.4 x 106 , or 0.8 x 106 cells/ml) and Scenedesmus subspicatus (0.05 x 106 , 0.1 x 106 , 0.2 x 106 , or 0.4 x 106 cells/ml) on culture of the water flea, Bosmina longirostris, were investigated. The experiment was carried out in a photoperiod of 16 h light:8 h dark at 25±1°C. At the beginning of the experiment, one B. longirostris individual (<24 h old) was put into each vessel, and the number of individuals and rate of population increase were determined for 30 days. Increasing the food density increased the number of individuals and the rate of population. The maximum number of B. longirostris individuals (7.1±2.08 ind/ml) and maximum rate of population increase (0.2±0.004/day) was in the group fed 0.2 x 106 cells/ml S. subspicatus. The effect on the number of the individuals was statistically significant (p<0.05).</p>