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The study on the dynamics of experimental population of freshwater rotifer Brachionus calyciflorus under exposure of different concentrations of methamidophos (0.01, 0.1, 1.0, 10.0, 100.0, 1000.0 and 10000.0 microg x L(-1)) and 17beta-estradiol (0.001, 0.01, 0.1, 1.0, 10.0, 100.0 and 1000.0 microg x L(-1)) showed that both methamidophos and 17beta-estradiol had significant effects on the mean growth rate of the population, but less affected the mean fertilization rate of mictic females. Methamidophos affected the mean ratio of ovigerous to non-ovigerous females and the mean mictic rate significantly but had less effect on the total resting egg production, while 17beta-estradiol was in adverse. Compared with the controls, both 10.0-10000.0 microg x L(-1) of methamidophos and 100.0 microg x L(-1) of 17beta-estradiol increased the mean population growth rate significantly. 0.1-10000.0 and 10.0 microg x L(-1) of methamidophos decreased significantly the average ratio of ovigerous to non-ovigerous females and the average mictic rate, respectively, and 1000.0 microg x L(-1) of 17beta-estradiol decreased the total resting egg production significantly. Within the range of test concentrations of methamidophos, the relationship between the mean population growth rate (Y, d(-1) and the concentration (X, microg x L(-1) of methamidophos could be described as Y = -2 x 10(-8) X2 + 0.002X + 0.374. The population growth rate could be used to assess the effects of methamidophos on the population dynamics of the rotifer.