The AAT isozymes of F_1 hybrids synthesized from inbred Citrus were analyzed, so that clarified the genetic system, and that made clear the structures of two loci, Aat-1 and Aat-2, and that had confirmed the estimated genotypes. In addition, the analysis of the alleles have suggested that evolution by gene duplication in Aat-2 locus on the homologous chromosome. The Aat-2 locus was consisted of the two gene loci which was at a distance.