The utilization of various agricultural pesticides has both direct and indirect effects on aquatic animals. The toxic effects of organochlorine insecticide endosulfan on tadpoles of the Chinese tiger frog (Hoplobatrachus chinensis) were investigated in this study. Specifically, we first examined the safe concentration (SC) and evaluated the toxic rank of endosulfan in an acute toxicity assay. We then examined the blood biomarker (erythrocyte nuclear abnormality) and determined the metabolic enzyme activitie of acid phosphatase (ACP), alkaline phosphatase (AKP) and lactate dehydrogenase (LDH) in a chronic toxicity test. In terms of the acute toxicity, the average mortality percentage of tadpoles significantly increased with increasing concentrations and exposure duration of endosulfan solution, indicating dose- and time-dependent lethality of this drug (Table 1). The half lethal concentration (LC50 value) of endosulfan for H. chinensis tadpoles at 96 h was 23.38 μg/L and the safe concentration was 2.34 μg/L (Table 2). Five different types of erythrocyte nuclear abnormalities were observed: broken nucleus, binucleated, unequal division, karyopyknosis and anucleated (Fig. 1). The total frequencies of abnormal erythrocytes were positively correlated with pesticide concentrations (Table 3). Activities of all three enzymes were influenced. Compared with the control treatment, the activities of both ACP and AKP were decreased with increased endosulfan concentrations, while LDH activity was first increased and then decreased (Table 4). These findings suggest that endosulfan has a high toxicity for H. chinensis tadpoles and that the erythrocyte nuclear abnormality and particular metabolic enzymes may be considered as biomarkers for environmental monitoring.