There have been few ecological studies on the effects of arsenic (As) on Enchytraeus crypticus. Effects on the morphology, survival, reproduction, growth, and internal concentrations of As in E. crypticus were investigated using the contact filter paper test and a standard toxicity test in both artificially spiked and naturally-contaminated soil. Oxidative stress was assessed by the antioxidant enzymes and malondialdehyde at concentrations that have non-lethal effects at different time intervals in a bioassay test. Arsenic affected the morphology of E. crypticus more with increasing pollution level and exposure time. Survival and reproduction tests in artificially and naturally contaminated soil showed large differences. Survival showed LC50 values in the artificially and naturally contaminated soil of 78.2 and 565 mg kg^-1, respectively. Reproduction showed EC10 values of 6.72 and 84.0 mg kg^-1 in the spiked and naturally polluted soil and corresponding EC50 values of 38.2 and 365 mg kg^-1, respectively. Body weight and body length of adults and juveniles showed similar trends of decline with increasing As concentration and the juveniles were more sensitive than the adults to As. Comparing the endpoints of the ecotoxicity tests on E. crypticus, reproduction was the most sensitive parameter followed by survival, body weight and body length. Internal As concentrations in E. crypticus are reported for the first time in the present study and increased with increasing As concentration in artificially spiked soil. Significant changes in the antioxidant defenses (CAT, POD and SOD) were induced, indicating oxidative stress at the lower As concentration (15 mg kg^-1) over the longer exposure time (8 days) or at the higher concentration (60 mg kg^-1) over the shorter exposure time (2 days). An increase in MDA with increasing exposure time and soil As concentration indicates oxidative damage. This study has investigated ecotoxic effects of soil As on E. crypticus in at different contamination levels, verified a series of biological parameters indicating soil As pollution, and provided information on the early diagnosis of soil As pollution using biological indicators.