Temperature and hydrostatic pressure are essential in determining the assemblage of species in their specific biotopes. To evaluate the effect of high pressure on the range of viability of thermophiles, the pressure and temperature dependence of the growth of the methanogenic archaebacterium Methanococcus thermolithotrophicus was investigated. High pressure up to 50 MPa enhanced the growth rate without extending the temperature range of viability. The optimum temperature remained unaltered (65 degrees C). Beyond 50 MPa, cell lysis predominated over cell proliferation. Destabilization was also observed at temperatures below and above the optimum growth temperature (<60 degrees C, >/=70 degrees C) and at low substrate concentrations.