The denitrifying bacterium strain MT-1 was isolated from the mud of the Mariana Trench. The optimal temperature and pressure for growth of this bacterium were found to be 30 degrees C and 0.1 MPa, respectively. However, it showed greater tolerance to low temperature (4 degrees C) and high hydrostatic pressure (50 MPa) as compared with denitrifiers obtained from land. From the results, it can be said that this organism is adapted to the environment of the deep sea. Strain MT-1 was shown to belong to the genus Pseudomonas by analysis of its 16S rDNA. The cytochrome contents of the bacterium were similar to those of Ps. stutzeri in spectrophotometric studies.