The effect of subthlethal concentrations of copper ions on respira-tory physiology of freshwater fish (Tilapia sp.) was studied. By comparing therespiratory frequency, respiratory amplitude and opercular movement among thegroups of fish reared in various concentration of Cu²⁺ for 15 days, it was foundthat the frequency of abnormal opercular movement increased with increasingconcentrations of copper ions, and the cough movement appeared ealier than theexpulision reflex did. This suggested that cough movement and expulsion reflexcould be used as the sensitive biological monitoring indexes under pollution ofsublethal concentration of copper ions.