We studied the effects of high temperatures and elevated hydrostatic pressures on the physiological behavior and viability of the extremely thermophilic deep-sea archaeon Thermococcus peptonophilus. Maximal growth rates were observed at 30 and 45 MPa although no significant increases in cell yields were detected. Growth at 60 MPa was slower. The optimal growth temperature shifted from 85 degrees C at 30 MPa to 90-95 degrees C at 45 MPa. Cell viability during the stationary phase was also enhanced under high pressure. A trend towards barophily at pressures greater than those encountered in situ at the sea floor was demonstrated at increasing growth temperatures. The viability of cells during starvation, at high temperature (90, 95 degrees C), and at low temperature (10 degrees C) was enhanced at 30 and 45 MPa as compared to atmospheric pressure. These results show that the extremely thermophilic archaeon T. peptonophilus is a barophile.