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The objective of this research was to determine the CE50; 48h of some of the herbicides and insecticides mostly used in paddy rice and to estimate the risks coefficient (deterministic analysis), classifying the chemicals toxicity on Daphnia magna. The following pesticides were evaluated: herbicides-oxyfluorfen, carfentrazone-ethyl, clomazone, quinclorac, pyrazosulfuron-ethyl, 2, 4–D, bispyribac-sodium, metsulfuron-methyl and bentazon; insecticides-lambdacyhalothrin, fipronil and carbofuran. The results showed that herbicides oxyfluorfen, oxadiazon and the insecticides lambdacyhalothrin, fipronil and carbofuran are the chemicals that present the highest toxicity to D. magna. The obtained CE50;

48h values in mg. L-1, were 3.18; 3.54; 12.252; 14.35; 134.31; 182.11; 407.3; 1,060; 2,100; 3,900; 0.00021; 0.15 and 0.32, respectively for oxyfluorfen, oxadiazon, carfentrazone-ethyl, clomazone, quinclorac, pyrazosulfuron-ethyl, 2, 4–D, bispyribacsodium, metsulfuron-methyl, bentazon, lambdacyhalothrin, fipronil and carbofuran. Considering the chemicals label rate dissolved in a 10 cm water depth in paddy rice, the herbicides oxyfluorfen, oxadiazon and the insecticides lambdacyhalothrin, fipronil and carbofuran deserve careful irrigation water managing.