Many deep-sea bacteria are specifically adapted to flourish under the high hydrostatic pressures which exist in their natural environment. For better understanding of the physiology and biochemistry of these microorganisms, properties of the glucose transport systems in two barophilic isolates (PE-36, CNPT-3) and one psychrophilic marine bacterium (Vibrio marinus MP1) were studied. These bacteria use a phosphoenol-pyruvate:sugar phosphotransferase system (PTS) for glucose transport, similar to that found in many members of the Vibrionaceae and Enterobacteriaceae. The system was highly specific for glucose and its nonmetabolizable analog, methyl alpha-glucoside (a-MG), and exhibited little affinity for other sugars tested. The temperature optimum for glucose phosphorylation in vitro was approximately 20°C. Membrane-bound PTS components of deep-sea bacteria were capable of enzymatically cross-reacting with the soluble PTS enzymes of Salmonella typhimurium, indicating functional similarities between the PTS systems of these organisms. In CNPT-3 and V. marinus, increased pressure had an inhibitory effect on a-MG uptake, to the greatest extent in V. marinus. Relative to atmospheric pressure, increased pressure stimulated sugar uptake in the barophilic isolate PE-36 considerably. Increased hydrostatic pressure inhibited in vitro phosphoenolpyruvate-dependent a-MG phosphorylation catalyzed by crude extracts of V. marinus and PE-36 but enhanced this activity in crude extracts of the barophile CNPT-3. Both of the pressure-adapted barophilic bacteria were capable of a-MG uptake at higher pressures than was the nonbarophilic psychrophile, V. marinus.