OECD Test Guidelines for Testing of Chemicals were used to reveal the effect of chemicals on the ecosystem. But it isn’t easy to apply the results of the test to the field because of environmental factors, such as temperature. Therefore, the examination is needed to consider the effects of environmental factors. In this study, we conducted the experiment to examine the effect of temperature to toxicants tolerance on Daphnia which is the one of test animals. The 48-h acute toxicity tests were carried out by using 2 or 4 days old single clone of Daphnia. To define the effects of temperature to toxic tolerance of Daphnia, 3 degrees (15, 20, 25 ^oC ) were set up. The test chemical was carbaryl. Before the tests, the body length was measured. Then, EC₅₀ values were defined. The EC₅₀ values of 2 days old individuals in 15, and 20 ^oC were 0.6 and 1.27 μg/L, respectively. The value in 25 ^oC was not obtained. The values of 4 days old ones15, 20 and 25 ^oC were 4.26, 4.62 and 5.12μg/L, respectively. The body length was elongated as rising temperature in 2 and 4 days old. The high EC₅₀ in high temperature might be due to large body length.