The microbial world of the deep subsurface environment located off the east coast of Taiwan, in the most southern part of the so-called Okinawa Trough, was studied using specific 16S rRNA oligonucleotides to target selected reducing and oxidizing bacteria. The sampling area consists of a cluster of volcanoes under an anoxic sea bottom. Sulfate-reducing bacteria were found to be dominant, by memberane hybridization with an SRB804 probe. Archaea was the second major group and was detected using an ARC915 probe. Very few nitrate-dependent ferrous iron oxidizers were identified in this extremely anoxygenic environment. Since a adaptation to a habitay by reducing or oxidizing bacteria depend upon the redox potential of an environment, the sulfate-reducing bacteria and Archaea found suggest that the collected sediments offer extremely anoxygenic conditions. Results further indicated the absence of oxygen at the hydrothermal vents of the volcano in the area investigated which shows that the volcano is dormant.