Biochemical genetic differences of two varieties of mitten crabs ( Eriocheir sinensis and Eriocheir jap onicus) from Liaohe, Huanghe, Changjiang, Oujiang, Pearl and Nanliujiang rivers in the mainland of China were analysed by acrylamide gel electrophoresis. The results showed: ( 1) There were no significant differences in biochemical genetic character of the mitten crab among Liaohe, Huanghe, Changjiang and Oujiang rivers, the genetic divergence among these four populations ( D = 0. 0006~ 0. 0053) were low and they all belong to populations of E . sinensis . ( 2) There was also no significant difference in biochemical genetic character of the mitten crab ( Eriocheir japonicus ) f rom Pearl and Nanliujiang rivers, the genetic distance was 0. 0007, which was lower than the genet ic divergence value at subspecies level. ( 3) There were significant dif ferences in the phenotype of SOD in the liver of the mitten crab between Liaohe, Huanghe, Changjiang, Oujiang rivers and Pearl, Nanliujiang rivers, where SOD - 2 was only found in the mitten crab of Liaohe, Huanghe, Changjiang , Oujiang rivers, while SOD- 4 was only found in the mitten crab of Pearl and Nanliujiang rivers, which could be used as biochemical genetic markers to differentiate these two groups of the mitten crab. The genetic distances between these two groups of the mitten crab were 0. 0721~ 0. 0747. ( 4) Dendrogram constructed by the genet ic distances among six populations of the mitten crab showed that there were two dif ferent clads. The northern clad was composed of the mitten crab ( Eriocheir sinensis) from Liaohe, Huanghe, Changjiang and Oujiang rivers, the southern clad consisted in the mitten crab ( Eriocheir japonicus ) from Pearl and Nanliujiang rivers.