Significant numbers of bacteria have been found in recent sediments wherever critical tests have been made for their presence. Total numbers as well as the kinds of microorganisms decrease with core depth. Several physiological types of bacteria together with fewer yeasts, molds and actinomyces have been found, some at depths exceeding 15 feet. The demonstrated ability of the microorganisms to function in sedimentary materials in the laboratory under conditions simulating those in nature indicates that they could be active in situ , and changes in certain properties of sediments suggest that the microorganisms have been active. Bacteria consume oxygen, and there are enough bacteria in sediments to account for the depletion of the dissolved oxygen. Similarly bacteria are probably responsible for the oxidation-reduction potential of sediments which ranges from Eh 0 to -0.5 volts whereas the overlying water is oxidizing in character. There are several ways in which the biochemical activities of microorganisms influence the hydrogen-ion concentration of sediments. Most types of organic matter are attacked by bacteria. The organic content and microorganisms influence the aggregation of particles. By altering the environmental conditions and by serving as a source of food, bacteria have a pronounced effect on the animal population of sediments. It is still indeterminate if bacterial activity plays a role in the genesis of petroleum, but there are several ways in which it might. The changes in the calcium, iron, manganese, sulfur, phosphate and carbonate content of sediments attributable to microorganisms are mentioned.