Manufacture of aromatic liquor differs from that of mellow liquor not only in the technology of fermenting and producing liquor but also in the technology of preparing leaven. The difference in the technology of preparing leaven results in the difference in the formation of bacterial groups in these two kinds of leaven. In this paper, bacteria in leaven are classified into three main groups: (1) acid-producing bacteria, (2) substratedecompocing bacteria and (3) Actinomyces. For each kind of leaven the numbers of each group of bacteria is counted and the quantitative composition and structural feature is studied. The relationship between these bacteria and the respective flavors of the two kinds of liquor is discussed.