Food quality can influence the growth and reproduction of zooplankton in freshwater systems.Food size,element ratios,biochemical composition and toxicity are indicative of food quality.The C/P ratios of Scenedesmus obliquus was used to represent the food quality in this experiment.We analyzed the effects of S.obliquus with different C/P ratios on the growth and reproduction of Daphnia similis.The C/P ratios of S.obliquus in four treatments were 881.8,512.3,124.3 and 42.4,respectively.The growth rates of D.similis in four treatments were 0.15,0.27,0.47 and 0.44 d^-1,respectively. D.similis did not release any offspring dur-ing this period of growth in treatment with C/P=881.8.The average numbers of offspring of first adult instar of D.similis in other three treatments were 1.2±1.3,9.8±2.3 and 9.5±2.5 ind./female,respectively.The results showed that the growth rate and off-spring number of D.similis decreased when the C/P ratio of S.obliquus was too high (512.3 and 881.8) or too low (42.4) com-pared with C/P=124.3.Our study indicated that different C/P ratios of food could affect the growth and reproduction of Daphnia,thus determining their survival in freshwater lakes.