The chronic effects of deltamethrin and endosulfan on non-target organisms have been investigated several times. However, most of these studies were only performed under certain environmental conditions and ignored the effects of food density. In the present study, the effects of deltamethrin (0.3, 0.6, 1.2, and 2.4 mg/L) and endosulfan (0.27, 0.54, 1.08, and 2.16 mg/L) on the population growth of the rotifer Brachionus calyciflorus were investigated at two densities of Scenedesmus obliquus (1.0×10⁶ and 3.0×10⁶ cells/mL). Compared with the blank control, high levels of deltamethrin and endosulfan significantly decreased the population growth rate and the maximum population density of B. calyciflorus, suggesting that these two chemicals are toxic to rotifers. In comparison to high algal density, the toxic effects of 2.4 mg/L deltamethrin and 2.16 mg/L endosulfan on rotifers were much more obvious at low algal density. The present results show the important role of algal density on influencing the toxicities of deltamethrin and endosulfan to rotifers.