Citrus species and their hybrids have been classified and identified on the basis of fruit shape, flower, leaf and tree characteristics. Recently, the zymogram method has been used some plants as an easy, reliable method to distinguish species or cultivars. Genetics of isozyme in Citrus leaves was widely used for the taxonomic investigation and breeding. Many native cultivars were grown in Japan. Analysis of Citrus groups may have a botanical significance and may provide valuable information on the Citrus breeding. Analyzing the two isozyme loci of aspartate aminotransferase (AAT; E. C. 2. 6. 1. 10), They detected that well recognized groups of Citrus cultivars. The zymograms were compared with the pollen and leaves on the heteroband of Aat-2 of AAT, and showed the disappearance of the hetero-band Aat-2. This paper descrided to clarify the dimeric nature of Aat-2.