Summary:Chemical changes resulting from microbial growth in a simple medium containing glucose as the only source of carbon, and ammonia as the only source of nitrogen, depended not only upon the concentration of nutrient materials and the rate of supply of air, but on the shape and size of the container, the depth of liquid and the method of aeration. If, by the time all the glucose was destroyed, the pH value had not fallen below about 3.0, continued aeration resulted in a substantial rise in pH value, due to oxidation to carbon dioxide of organic acids formed in the earlier stages.Initial growth in a simple medium was suppressed not only by aeration of a pure culture with a current of CO2‐free air, but also by violent aeration of a mixed culture with air with a normal content of carbon dioxide.Changes occurring in a complex organic medium, a sterile extract of red beet, were similar in character to those observed in the simple medium, but were much less affected by irregularities in growt