Abstract: In this study, the acute toxicity of lambda-cyhalothrin, a synthetic pyrethroid pesticides heavily used in the paddy fields, to the juvenile and adults of Astacus leptodactylus was investigated. The two different experiments, one for the juvenile (0.417±0.001 g) and one for the adult (39±2.2 g) crayfish were conducted. Acute toxicity tests was performed in 3 L glass jars for the juvenile and in aquariums for the adults. Acute toxicity tests with a static control and 6 different concentrations with 3 replications were tested. About 10 healthy individuals were placed in each. During the experiments, the water temperature was kept at 20°C and average pH was constant at 7.8. At the end of the 96 h experiments, the LC50 value of lambda-cyhalothrin insektisitinin for Astacus leptodactylus were found as 0.84 ng L-1 for the juveniles and as 0.207 mg L-1 for the adults. As a result, the heavily used lambda-cyhalothrin in the paddy fields was determined to be very toxic for juveniles and for adult of Astacus leptodactylus.