Using RT-PCR method, the glutathione transferase Pi cDNAs were cloned from Cyprinus carpio, Hypophthalmichthys molitrix, and Carassius auratus. The open reading frames (ORFs) from the 3 fishes were 627 bp long (encoding for 208 amino acids) with the initial code ATG and the terminal code TGA. The sequence similarity was 50% between fish and mammals, 33% between fish and amphibian, and 15% between fish and arthropoda, respectively. The sequence similarity was big among fishes, and the average value of the 4 cyprinids was about 85%. Phylogenetic tree was constructed for 13 species based on GST Pi amino acid sequences using MP (Maximum Parsimony) method. Two major clusters were recog-nized: cluster one consisted of Mammals (bootstrap 100) and cluster two consisted of fishes (bootstrap 93). Based on the sequences analyses of N/C domain of GST Pi, we proposed the detoxification mechanism of freshwater fishes that were thought to have stronger tolerance to microcystins.