Abstract: Effects of sublethal levels(0.01, 0.1, 1.0, 10.0, 100.0, 1000.0 and 10000.0μg/L) of organophosphorus insec ticides includingmethamidophos(O, SDimethyl phosphoramidothioate, CAS No.10265-92-6) and acephate(O, S-Dimethy. acetylphosphoramidothioate, CAS No130560191) on population dynamics of rotifer B rachionus calycif lorus were studied by 3 day population growth test at 25±1℃, in dark, and with 3.0×106 cells/mL of S.obliquus as the rotifers’food.The results showed that both methamidophos and acephate influenced significantly the population growth rate, the ratio mictic females/amictic females and the mictic rate of the rotifers.Methamidophos influenced markedly the carryingegg females/non carryingegg females of the rotifers, but acephate did not1Compared to the controls, 100.0μg/L methamidophos and 1.0-10000.0μg/L acephate increased significantly the population growth rate, but the reverse was true for 1, 000.0 and 10, 000.0μg/L methamidophos. 1,000.0μg/L methamidophos increased significantly the ratio carryingegg females/non carryingegg females10.1μg/L methamidophos and 10.0-10000.0μg/L acephate increased significantly the ratio mictic females/amictic females, and both 0.1 μg/L methamidophos and 10 μg/L acephate increased significantly the mictic rate.10.0-10000.0μg/L acephate significantly increased the resting egg production.The abovestated results indicated that sublethal levels of methamidophos and acephate influenced significantly the population dynamics ofB.calyciflorus.