The soy koji mold Aspergillus sojae is purely cultured on the materials of soy koji, and soaked in the sterile concentrated sodium chloride solution (26%). To it, Ped. soyae and Sacch. rouxii are aseptically inoculated and their roles playing in soy sauce brewing are examined. Bacillus subtilis is added in the form of wheat bran culture into the purely cultured soy mash and its influencies are observed. The appropriate addition of the bran culture of Bac. subtilis favors for the development of necessary aroma, and eliminates the polypeptide-like turbidity in soy sauce. The growth of Ped. soyae in soy mash stimulates the formation of organic acids, and makes the mash acidic which is necessary for the sound fermentation, removes undesirable flavors, and adds the indispensable good flavors for soy sauce. But the rapid lowering of pH results in the depression of Aspergillus Alkaline Proteinase activities, followed by its inactivation, thus the percentage of solubilization of the protein material decreases. The colour of soy sauce also becomes lighter. Sacch. rouxii produces ethanol, and cooperating with Ped. soyae, plays an important part for developing the soy sauce flavor. Asp. sojae is the most important microorganism for soy sauce brewing. Almost all the essential enzymes for soy sauce brewing originates in Asp. sojae, as a result, fundamental chemical components, aroma and flavor are developed by enzymes from this mold. The soy sauce brewed by Asp. sojae alone, however, is not equal to the one which is made with mixed organisms, but it is only the mother body of soy sauce. The solubilization ratio of the protein material has also reached to the maximum without the aid of other microorganisms. However, the production of organic acids and ethanol is insufficient, polypeptide-like turbidity in soy sauce has remained, and aroma and flavor are not as well as the normally brewed soy sauce.