To explore the toxic effects of several heavy metals on hemocytes from , we evaluated the cell viability (CV) and esterase activity (EA) of the -3 mol/L heavy metals (Cd²⁺, Hg²⁺, Cu²⁺, and Zn²⁺, compared with a control group without exposure to heavy metals) for 6 h by flow cytometry. The results showed no significant effects of 10^-9-10^-6 mol/L Cd²⁺ on CV and EA. 10^-5 mol/L Cd²⁺ significantly inhibited EA. Both CV and EA significantly decreased after exposure to 10^-4 mol/L and 10^-3 mol/L Cd²⁺. There were no significant effects of 10^-9-10^-6 mol/L Hg²⁺ on CV and EA, whereas 10^-4 mol/L and 10^-3 mol/L Hg²⁺ significantly decreased CV and EA. Concentrations of 10^-9-10^-5 mol/L Cu²⁺ had no significant effects on CV and EA; however, 10^-4-10^-3 mol/L Cu²⁺ significantly decreased CV and EA. Although 10^-9-10^-5 mol/L Zn²⁺ had no significant effects on CV and EA, 10^-4 mol/L Zn²⁺ significantly decreased EA, whereas 10^-3 mol/L Zn²⁺ significantly decreased CV and EA. The toxic critical concentrations of Cd²⁺, Hg²⁺, Cu²⁺, and Zn²⁺ were 10^-4 mol/L, 10^-5 mol/L, 10^-4 mol/L, and 10^-3 mol/L, respectively. The toxicity of these four heavy metals on > Cd²⁺ > Cu²⁺ > Zn²⁺. The toxicity of heavy metals on hemocytes was dose-dependent. EA is more sensitive to heavy metal stress, and could be used as an indicator for toxicology studies.