Abstract: The aim of this research is to discuss the acute and chronic toxic effects of tetracycline (TC) and tetracycline hydrochloride (HTC) on Paramecium caudatum , and preliminarily assess the potential threat of tetracycline antibiotics to aquatic organisms. 24h- lethal concentration 50% (LC 50) and the lowest observed effect concentration (LOEC ) of TC and HTC onParamecium caudatum were tested.LOEC was used in the stress test. 15-day monoclonal growth curve ofParamecium caudatum was investigated. The results were as follows: 24 h-LC 50 of TC was 52.36125 mg/L andLOEC was 15.19917 mg/L. 24 h-LC 50 of HTC was 50.24583 mg/L, LOEC value was 24.67175 mg/L. 15- day chronic toxic test showed that the two kinds of antibiotics had significant inhibitory effects on 15-day monoclonal growth curve ofParamecium caudatum . The increase rate of population declined and the numbers of individuals reduced. These results suggested that tetracycline and tetracycline hydrochloride had acute and chronic toxicity on single-cell animal. We should make a definite standard for using and emission, and monitor and control the pollution of TC and HTC.