The prokaryotic community inhabiting the deep subsurface sediments in the Forearc Basin of the Nankai Trough southeast of Japan (ODP Site 1176) was analyzed by 16S rDNA sequencing. Sediment samples from 1.15, 51.05, 98.50 and 193.96 m below sea floor (mbst) harbored highly diverse bacterial communities. The most frequently retrieved clones included members of the Green non-sulfur bacteria whose closest relatives come from deep subsurface environments, a new epsilon-proteobacterial phylotype, and representatives of a cluster of closely related bacterial sequences from hydrocarbon- and methane-rich sediments around the world. Archaeal clones were limited to members of the genus Thermococcus, and were only obtained from the two deepest samples. (C) 2003 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.