Soluble Glutathione S-transferase (sGST) of freshwater fish is extremely important to microcystin purification from fish body, and therefore is also named microcystin-detoxifizyme.PCR using two degenerated primers, yielded a cDNA fragment of 405 bp from the liver of bighead carp(Aristichthys nobilis).This cDNA fragment was completed by 5- and 3-RACE.The complete sGST cDNA sequence of bighead carp was 934 bp in length, containing an open reading frame of 672 bp (encoding 223 amino acids), flanked by 104 bp 51UTR and 158 bp 3-UTR.The deduced amino acid sequence from this sGST cDNA fragment contains two conserved domains, N-terminal domain (glutathione-binding site) and C-terminal domain (substrate-binding site).Comparison of the N-terminal domain and C-terminal domain of bighead carp sGST with rock bream (Oplegnathus fasciatus), red sea bream (Pagrus major), chicken, mouse, rat and human sGST showed that, the N-terminal domain is highly conserved (75%) in the sGST of fishes, Aves and mammals, while the C-terminal domain has less similarity (43.6% -55.9%) among different sGST, which is consistent with the different function of the two domains.From the homologous analysis result, we might conclude that, there is a big difference in the C-terminal substrate-binding site between the microcystin-detoxifizyme gene of freshwater fish and the glutathione S-transferase gene of mammals, which seems to be consistent to the special function of this gene in freshwater fish for microcystin detoxification.