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Rotifers due to their relatively short lifespan, high fecundity and high rate of population growth are ideal for chronic toxicity tests. The aim of this research was to determine the lethal concentrations (LC50 24 h) of deltamethrin and imidaclopride and their impacts on the reproduction and growth of the freshwater rotifer, Brachionus calyciflorus. Experiments were carried out based on the guidelines of the standard methods of OECD. Based on LC50 levels of pesticides, different concentration treatment groups designated and rotifer population responses in the five different concentrations of deltamethrin (0.00, 0.05, 0.10, 0.21 and 0.53 mg L-1) and imidacloprid (0.00, 6.22, 12.45, 24.91 and 62.27 mg L-1) were studied during ten days. The LC50 24h of deltamethrin and imidacloprid for freshwater rotifer determined as 1.06 mg L-1 and 124.54 mg L-1, respectively. The density of rotifers in all treatment groups of pesticides decreased significantly compared to the control group at tenth day (p<0.05). The ratio of ovigerous females to nonovigerous females (OF/NOF) and the ratio of mictic females to amictic females (mic/amic) were significantly affected (p<0.05) in all concentrations of both insecticides. The results of this study suggested that B. calyciflorus are severely sensitive to deltamethrin rather than to imidacloprid pesticide.