Bacteria, yeasts, molds, and allied microorganisms of many physiological types are widely distributed in the sea. They are particularly abundant in bottom deposits where they influence the diagenesis of sediments in many ways. By virtue of their ecological versatility, their ready adaptability to various environmental conditions, their rapidity of multiplication under favorable conditions, and their ability to catalyze reactions involving nearly all kinds of organic compounds as well as several inorganic compounds, marine bacteria are important biochemical, geological, and hydrobiological agents. Bacteria affect the abundance of other organisms in the sea by producing plant nutrients, by competing for dissolved oxygen, by serving as a source of food for numerous genera of animals, by affecting physicochemical conditions in localized regions, and by infecting both plants and animals. The numbers of bacteria found in sea water, as determined by plating procedures which detect only a small percentage of viable cells, range from a few cells to hundreds of thousands of cells per ml. The numbers and kinds of bacteria in the sea is influenced by the organic content of water, the presence of suspended solids to which bacteria can attach, the occurrence of other organisms, and a multiplicity of other interrelated factors. While temperature affects the rate of bacterial activities, it seems to have only an indirect effect upon the distribution of bacteria in the sea. Neither low temperatures nor high hydrostatic pressures, characteristic of the deep sea, restricts the bacterial population. The low concentration of organic matter appears to be the...