Abstract: In order to understand the acute toxicity of alizarin red S to Aspiorhynchus laticeps juvenile fish,the A.laticeps juvenile fish with body weight of(5.8±0.6)g was studied at 10℃ using a semi-static test method.The results showed that,when the concentration of alizarin red S was 400 mg/L,the mortality rate at 96 hours was 0;when the concentration of alizarin red S was 900 mg/L,all died within 72 hours;result showed the semilethal concentrations of alizarin red S on Aspiorhynchus laticeps at 24 hours,48 hours,72 hours,and 96 hours were respectively 788,718,651 and 618 mg/L.The safe mass concentration was 179 mg/L.Our results indicated that alizarin red S had low toxicity to Aspiorhynchus laticeps,which was suitable for tagging and releasing of Aspiorhynchus laticep.In the practical work,on the basis of the guarantee survival rate,our results suggested selecting concentrations between 179 mg/L and 400 mg/L as tags to get better marks.