The toxicity of five herbicides, Propanil, Machete, Rilof-H, Saturn, and Ronestar, commonly used in Iranian rice fields, were investigated in acute toxicity bioassays on two organisms belonging to different in aquatic food chain. Compared with Daphnia magna, green algae Selenastrum capricornutum, showed more sensitivly to the herbicides. Machete and Saturn were the most toxic herbicides for algae. Three other herbicides, having more effect on the toxicity rate were Ronestar, Rilof-H, and Propanil. The toxic herbicides for Daphnia magna were Rilof-H and Saturn, and the others showed less toxicity. Comparing the amount of 6 to 23 mg/1 field concentrations of the herbicides with 0.01 to 12.46 mg/1 MAV-values for Selenastrum capticomutum and 0.446 to 9.5 mg/1, MAC-values for Daphnia magna in laboratory experiments revealed that the herbicides would certainly kill all useful organisms in aquatic food chain such as algae and zooplanktons in rice fields, and would damage other neighboring ecosystems in rivers, reservoirs, lagoon and the Caspian Sea which are connected with the fields. Ministry of Agriculture should be asked to introduce other less toxic herbicides.