Abstract: The clams Meretrix meretrix and Mactra veneriformis were used as experimental objects and cultured in seawater containing petroleum hydrocarbons.The petroleum hydrocarbon concentrations of seawater were 0.1 mg/L, 0.3 mg/L, 1.0 mg/L and 3.0 mg/L separately, with sampling occurring at day 0, 0.5 1, 3, 6, 10, 15.The glutathione s transferase GST and superoxide dismutase SOD activities were determined.The results showed that GST and SOD activities of two tissues were higher than controls in 0.1 and 0.3 mg/L exposure groups(P < 0.05).GST and SOD activities all increased first and then were restrained in later time except that the gills GST activities of clam Mactra veneriformis were restrained over whole time in 1.0 mg/L groups.The SOD and GST activities were all restrained over whole experimental time except that SOD activities of digestive gland of clam Meretrix meretrix increased in early time in 3.0 mg/L groups(P < 0.05).Through comparative studies, SOD and GST activities rise rats of clam Mactra veneriformis were higher than clam Meretrix meretrix in early time in 0.1 and 0.3 mg/L groups, and then were lower than Meretrix meretrix in last experimental time.The enzyme activities inhibition rates of clam Mactra veneriformis were higher than clam Meretrix meretrix in 1.0 and 3.0 mg/L groups(P < 0.05).The results indicated that clam Mactra veneriformis were more sensitive than clam Meretrix meretrix under petroleum hydrocarbons exposure, so the clam Mactra veneriformis can be used as the petroleum hydrocarbons pollution-indicating organism firstly.The tissue gills changed more significantly than digestive gland, so the gills can be used as the priority indicator tissue.The SOD and GST activities changes showed the same trend, therefore, the two biomarkers can be combined to evaluate the risk of petroleum hydrocarbons and other organic pollutants in the later researches and practice.