The AAT isozymes of F_1 hybrids synthesized from inbred Citrus and triforiate were analyzed in order that might clarify the estimated genotypes of species and cultivars. The results analyzed AAT isozyme showed to recognize the genotypes of Aat-1 in Citrus and its relatives, and that clarified gene duplication Aat-2 had acquired distinctive functions in Citrus, but the duplication of gene locus in Poncirus was not confirmed.