Thirty species of marine bacteria were examined for their ability to grow and reduce nitrate at increased hydrostatic pressures. All grew and reduced nitrate at pressures up to 300 atm, but not as rapidly as at lower pressures. Only 21 species grew in nutrient medium compressed to 400 atm, 11 grew at 600 atm, and 8 grew and reduced nitrate at 1,000 atm. Washed cells, grown at 1 atm, of all species reduced nitrate at deep‐sea pressures, the average rate being only about one‐fifth as fast at 1,000 atm as at 1 atm.The nitrate‐reducing ability of washed cell suspensions of Pseudomonas perfectomarinus slowly diminished during storage, more slowly at 2C than at 30C. At 2C, such diminution was more rapid at 1,000 atm than at 1 atm, thereby indicating that high pressure inactivates the nitrate‐reducing enzyme system of Ps. perfectomarinus. Subjecting washed cells of this organism suspended in oxygen‐free seawater to pressures of 1,000 to 1,800 atm for a few minutes had no measurable effect on their ability to reduce nitrate after decompressing them to 1 atm, but holding such cells for several hours at high pressures inactivated their nitrate‐reducing enzyme system. Such inactivation was complete and irreversible after holding the cells for 24 hr at 10C at 1,800 atm. Enzyme inactivation by increased pressures was more rapid at low temperatures, 2C, than at higher temperatures (tests made at 10, 21, and 3OC).