Hydrostatic pressure in the sea, which increases approximately 0.1 atmosphere per meter of depth, appears to be a factor that influences bacterial activities. This was indicated by the observations of Certes (1884a,b), Certes and Cochin (1884), and Regnard (1884a,b). The reports of Chlopin and Tammann (1903), Hite et al. (1914), Larson et al. (1918), and more recent ones reviewed by Macheboeuf et al. (1933), Cattell (1936), and Basset et al. (1938) indicate that the critical pressure tolerance of microorganisms ranges from 3,000 to 12,000 atmospheres, which is higher than pressures characteristic of the greatest known depth of the sea, the latter being of the order of 1,100 atmospheres. The apparent discrepancies are probably due to differences in experimental conditions, including growth phase, species, culture medium, temperature, time, and other factors now known to influence the pressure tolerance of microorganisms. ZoBell and Johnson (1949) found that many common microorganisms from surface soil and shallow water were killed in the logarithmic phase of growth in 48 hours at 30 C by pressures of only 500 to 600 atmospheres, and multiplication of some microorganisms was inhibited at 300 atmospheres. Resting cells, endospores, and bacteria from the deep sea were found to be more pressuretolerant as were organisms pressurized at higher temperatures. The term "barophilic" was coined to describe organisms that grow preferentially or exclusively at high hydrostatic pressures. Such bacteria have been found in the deep sea and also in brines from deep oil wells. Their counterparts that grow only at relatively low pressures, say up to 200 atmospheres, are described as "barophobic." Those that tolerate high pressures but grow best at normal pressures are described as "baroduric." The present paper is concerned primarily with observations on the occurrence and behavior of barophilic, barophobic, and baroduric bacteria in the sea. METHODS Bacterial cultures in the logarithmic phase of growth were transplanted to nutrient broth in small test tubes, size 10 X 50 mm, each having a sand-blasted area for numbering with a pencil. The nutrient broth used for this purpose consisted of 0.5 per cent peptone, 0.5 per cent yeast extract, and 0.01 per cent ferric phosphate in aged sea water (ZoBell, 1946). Following autoclave sterilization its reaction was pH 7.5. The medium was inoculated with enough cells to give from 100 to 1,000 per ml. The tubes were stoppered with no. 000 neoprene 1 Contribution from the Scripps Institution of Oceanography, New Series 491. This paper is a contribution from the American Petroleum Institute Research Project 43A.