This experiment investigated the effects of dissolved oxygen (DO) on the survival and immune responses of scallop Chlamys farreri. The scallops (initial mean dry weight of soft tissue 1.52+/-0.10 g) were cultivated in the seawater with different DO levels (8.5, 6.5, 4.5, and 2.5mg l(-1), respectively) for 21 d. Each treatment had triplicate groups of 35 animals. During the experimental period, the scallops were fed with Spirulina maxima, and water temperature ranged from 15.2 degrees C to 17.5 degrees C, salinity from 29.5 per thousand to 32.5 per thousand and pH from 7.5 to 8.2. Survival, specific growth rate (SGR) and total haemocyte count (THC) were examined at the end of the study, and superoxide dismutase (SOD), acid phosphatase (ACP) and alkaline phosphatase (ALP), were examined at 12 h, 24 h, Day 7, Day 14 and Day 21 after being exposed to the graded DO levels. The lower DO levels (2.5 and 4.5mg l(-1))resulted in lower survivals of scallops, and the survival (81.7%) at 2.5mg 1(-1)DO was significantly lower than those (100.0%) at 8.5 and 6.5mg l(-1) DO. Similarly, the SGR and THC of scallop gradually reduced with decreasing DO levels, and reached significant levels at 2.5mg l(-1) DO (P<0.05). At higher DO levels (8.5 and 6.5mg l(-1)), the SOD activity maintained rather stable during the entire sampling period. At lower DO levels (4.5 and 2.5mg l(-1)), however, the SOD activity significantly increased at 12 h, and then significantly decreased to the levels below the normal. At the two lower DO levels, ACP activities had no significant changes before Day 7, and then declined to the levels that were significantly lower than the normal. Significantly higher ALP activity was only observed at 12 h in the treatment of 2.5mg l(-1) DO, but in all other treatments and sampling times it fluctuated in a narrow range. In conclusion, less than 4.5mg l(-1) DO reduced the survival and depressed the immune responses of C. farreri.