A LTHOUGH it is frequently stated in the popular as well as in some scientific literature that the ocean floor beyond the continental shelves is barren of bacterial life, primarily due to the lack of food and the prevailing low temperature and high hydrostatic pressure, investigators have almost invariably demonstrated the presence of an abundant bacterial flora in bottom deposits wherever critical analytical procedures have been applied. In fact, there are usually from ten to ten thousand times as many viable bacteria per gram of mud as there are in a corresponding quantity of water in any superimposed strata. While nearly all of the studies on marine bacteria to date have been made in relatively shallow water near land, a few workers have recovered significant numbers of living microorganisms from mud collected at depths of a mile or more. However, the recovery of organisms from such depths is by no means incontrovertible proof that these organisms are biochemically active in such an environment, because they may be merely passive inhabitants which have settled from above and have been preserved by the cold. Inasmuch as over four-fifths of the ocean floor exceeds one mile in depth and has a temperature colder than 30C., it is of importance to know if bacteria and kindred microorganisms survive and if they are physiologically functional under such conditions. Our knowledge of the activities of bacteria in the sea is very fragmentary, but the few investigations which have been made indicate that bacteria probably play an important role in the sea. They themselves are consumed as food by many small marine animals and, also, by their activities they produce plant nutrients. They are the responsible agents for many of the chemical and physico-chemical changes which occur in sea water or on the ocean bottom, and they may be of geological importance (cf. Bavendamm, I93z); Benecke, I933; Waksman, I934). Experimental evidence indicates that for practical purposes the hydrostatic pressure of deep water is not inimical to microbial well-being. Tremendous pressures are encountered on the sea bottom but bacteria have been shown to tolerate far greater pressures than occur even in the most abyssal depths. Chlopin and Tammann (I903) found that pressures up to about Zgoo atmospheres, or approximaitely three times that found in the ocean, failed to harm bacteria, yeasts or molds, although individual organisms differed greatly in their susceptibilities to higher pressures. In the experiments of Larsen, Hartzell, and Diehl (I9I8), non-spore-formers survived at 3000 to 6ooo atmospheres and then, at these pressures, compressed gases, such as carbon dioxide, or the sudden release of pressure were found to be more detrimental than pressure itself. The actual recovery of bacteria by Certes