Abstract: In this study Daphnia magna was used as test animals. Daphnids were exposured for 96 h to DDVP concentrations ranging from 0.001-0.5 ppm. Death daphnids were counted under a stero microscope 24, 36, 48, 72 and 96 h intervals. The values of lethal concentration (LC10, 50, 90) and Lethal time (LT10, 50, 90) associated 95-99% confidence limits were calculated. Mortality rate between each dosages and mortality rate between 24-96 h depended on time were correlated. LC50 values were changed between 0.741-0.035, while LT50 values were found between 158 17- 4 25. Mortality ratios of Daphnia magna treated with malathion during 96 h period was found significant at 95-99% level (p<0.001-0.005).