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The bactericidal action of sea water was measured as the difference in survival of cells of Escherichia coli in untreated and autoclaved portions of water samples. The beneficial effect of sterilization by heat on the survival of E. coli in sea water varied with season and was most marked during summer months, however, the magnitude of the effect differed greatly from sample to sample. The more obvious and commonly suggested explanations for the bactericidal action of sea water were tested experimentally. The pH and salinity of sea water were changed by autoclaving, but the direction of the former was detrimental rather than beneficial and the significance of the latter was not clarified. The survival of cells of E. coli in filtered portions of some water samples was greater than that in untreated portions and equal to that in autoclaved portions, indicating that predators and competitors removed by filtration had contributed significantly to the rapid death of the bacterium in the untreated water. However, in the majority of samples tested, survival of E. coli in autoclaved water was considerably greater than survival in filtered water. The possibility that the beneficial effect of autoclaving over and above that of filtration resulted from inactivation or destruction by heat of bacteriophages and thermolabile toxic substances such as antibiotics was considered. Moreover, the suggestion was tested that the increased survival of E. coli in autoclaved sea water was due to the ability of heat to disrupt and degrade microbial cells and thermolabile compounds and, thereby, to cause an increase in concentration of available nutrients in sea water. It was concluded that the bactericidal action of sea water is not explicable in terms of the destruction or inactivation by heat of bacteriophages or antibiotics. Although added organic matter influenced the survival of E. coli, the test organism was not an effective competitor in sea water and the nutrient levels required to offset the bactericidal action were excessive. Artificial sea water was demonstrated to exert a bactericidal action comparable to that of natural sea water. Low levels of cysteine which favor survival of E. coli in natural sea water had a similar effect in artificial sea water. Nevertheless, it is not at this time possible to conclude that the factors responsible for the bactericidal action of artificial sea water are identical with those responsible in natural sea water.