Biochemical genetic characteristics of GIFT (Genetic Improvement of Farmed Tilapia) strain, "EGYPT" strain, "78" strain, "88" strain and "AMERICA" strain of Nile tilapia (Oreochromis niloticus) were analysed by polyacrylamide gel electrophoresis.The result showed that there were significant differences of phenotype of Est in the liver among different strains. Est-2 was only found in GIFT strain and "EGYPT" strain, which could be used as a genetic marker to differentiate these strains of Nile tilapia.