The acute toxicity of some insecticides (Fenpropathrin, Dichlorvos and Methidathion) was investigated on Artemia sauna (L. 1758) and Daphnia magna (Straus 1820) which are both living crustaceans in aquatic system. Insecticides are widely used in Turkey, because of the climatic and ecological characteristics of the regions which are suitable for the improvement of pests, diseases and weeds. In the present study, these aquatic organisms were exposed for 24 h to Fenpropathrin, Dichlorvos and Methidathion concentrations ranging from 1-50 ppm. Death organisms were counted under a stereo microscope after 24 h. The values of lethal concentration (LC(50)) associated with 95-99% confidence limits were calculated. Mortality rate between each dosage of each insecticide for both organisms (Anemia sauna and Daphnia magna) at 24 h were compared. LC50 values of Fenpropathrin, Dichlorvos and Methidathion for Anemia saiina were determined to be 24.413, 21.634 and 2.075 while those found for Daphnia magna were 22.580, 25.432 and 47.606, respectively. Results indicated a significant toxicity of Methidathion on Anemia sauna, while Daphnia magna proved to be the least sensitive.