The distribution of microorganisms in the subsurfaces of hydrothermal vents was investigated by using subvent rock core samples. Microbial cells and ATP were detected from cores taken at depths of less than 99.4 and 44.8 m below the seafloor (mbsf), respectively. Cores from various depths were incubated anaerobically with a heterotrophic medium. Growth at 60 and 90°C was ascribed to a Geobacillus sp. in the 448.6-to 99.4-mbsf cores and a Deinococcus sp. in the 64.8-to 128.9-mbsf cores, respectively, based on the 16S ribosomal DNA analysis.Since the discovery of deep-sea hydrothermal vents, a number of thermophiles and hyperthermophiles have been isolated from chimneys, sediments, and ambient water of the hydrothermal vent fields (reviewed by Reysenbach et al. [21]). In addition, non-culture-dependent 16S ribosomal DNA (rDNA) analysis has been applied to a variety of vent samples (see, e.g., references 11, 14, 15, 23, 29, and 30). However, most of the previous studies were limited to the surfaces of hydrothermal vent systems, while interest in the subsurface habitats of hydrothermal vents (subvents) has been increasing.Only a few subvent microbiological studies have been conducted with sediment, sedimentary rock layers, and igneous rocks from relatively shallow depths (less than 52 m below the seafloor [mbsf]) (4,5,22,26,27). Here, we report the first evidence for the occurrence of a deep-sea subvent biosphere (maximum depth, 128.9 mbsf), by using igneous rock core samples from a back-arc basin hydrothermal vent field.