Abstract: Coumoxystrobin is a strobilurin fungicide developed independently in China. In order to clarify the toxicity of coumoxystrobin to aquatic organisms, the acute toxicity and the chronic toxicity of coumoxystrobin on Scapholeberis mucronata were determined by the semi-static test method. The results of acute toxicity tests indicated that coumoxystrobin was highly toxic to S. mucronata, and the 48 h-EC₅₀ value was 202 μg/L. Results of the chronic toxicity tests showed a significant dose-effect relationship, including the first brood time, the number of offspring per adult, the number of broods and the average 21 d body length of survivors of the S. mucronata exposed to coumoxystrobin. The number of cumulative molts and the size of the first brood of S. mucronata were not significantly influenced by the exposure to coumoxystrobin. The 21 d no observed effect concentration (NOEC) of coumoxystrobin was 0.22 μg/L for S. mucronata and the lowest observed effect concentration (LOEC) was 0.67 μg/L on the basis of breeding amounts. The results showed that coumoxystrobin had inhibiting effects on the growth and reproduction of S. mucronata. The results provide a reference for the reasonable evaluation of the risk of coumoxystrobin to aquatic ecosystem.