Abstract: Superoxide dismutase (SOD) scavenges superoxide anion free radicals in organisms and effectively prevents the damage of oxygen free radicals to organisms. In the present study, the full-length cDNAs of two SOD genes: mitochondrial manganese superoxide dismutase (SpmtMnSOD) and cytoplasmic manganese superoxide dismutase (SpcytMnSOD), were cloned from the gonads of Scylla paramamosain. The full-length cDNA of SpmtMnSOD is 1 154 bp, encoding a polypeptide of 218 amino acids (aa) that contains 16 aa signal peptides targeting mitochondria; the fulllength cDNA of SpcytMnSOD is 1 184 bp, which encodes a polypeptide of 286 amino acids without signal peptide sequence. Both two MnSODs contain the tag sequence of manganese-type SOD, as well as four conserved manganese ion binding sites. Quantitative real-time PCR was used to detect the expression of MnSODs gene in various tissues, gonadal development stages, and under different stresses. These results showed that the SpmtMnSOD and SpcytMnSOD were mainly expressed in the heart, gill, ovary and hepatopancreas, which are metabolically active or involved in the immune response. It is speculated that MnSODs play an important role in coping with oxidative stress and immune response. SpmtMnSOD and SpcytMnSOD were significantly higher in O6 (tertiary vitellogenesis) than those of other ovarian developmental stages, and their expression levels were low during the testis development. It is also speculated that they are involved in the scavenging of free radicals during ovarian maturation. After infected by Vibrio parahemolyticus, compared to the control group, the expression of SpmtMnSOD increased in gills (12 h, P<0.05), and the expression of SpcytMnSOD significantly increased in hepatopancreas (12 h) and gills (3 h, 12 h, P<0.05). After the challenge with peptidoglycan (PGN), compared to the control group, the expression of SpmtMnSOD was significantly increased in the hepatopancreas (3, 6, 12 h, P<0.05), and the expression of SpcytMnSODwas significantly increased in the hepatopancreas (3, 6, 12 h), gills (6, 24 h) and haemocytes (6, 12 h, P<0.05). After the challenge with polyinosinic:polycytidylic acid (Poly I:C), compared to the control group, the expression level of SpmtMnSOD in hepatopancreas (6, 12 h), gills and haemocytes (3, 6 h) and the expression level of SpcytMnSOD in hepatopancreas (12 h), gills (6, 12 h) and haemocytes (3, 6 h) were all up-regulated (P<0.05). It is thus speculated that they participate in the stress response to the external environment stimulus, balance the redox system in the body, and play an important role in removing free radicals generated by immune stress in mud crabs.