The hydrostatic pressure of sea water, which increases approximately 0.1 atmosphere per meter of depth, was found to affect the viability, reproduction, and morphology of 63 stock cultures of marine bacteria representing several genera. Many of the cultures were killed at 27° C by pressures ranging from 200 to 600 atm, although some few reproduced at 600 atm. Initial inoculum concentrations of the various bacteria appeared to influence their ability to reproduce or to tolerate high pressures. Pressures exceeding 400 atm inhibited the fission of certain bacteria without stopping their growth, thereby resulting in bizarre cells, some of which formed Jong filaments.