A borehole core from 20 m depth of a Japanese landfill was characterized chemically and microbially. The borehole core sample was typically divided into 5 waste layers; 2.4-4.0 m, 5.7-8.5 m, 925-9.6 m, 9.77-14.9 m, and 15.9-17.86 m depths. The waste layers' ages spanned about 14 years between the bottom and top. Archaeal 165 rRNA gene and eubacterial 165 rRNA gene in the waste samples at their respective levels were 9.8 x 10(5)-7.2 x 10(7) and 1.2 x 10(7)-7.2 x 10(9) copy/g-wet. Similar to populations of viable and culturable bacteria, those populations were high at 7.0 m and 17.5 m depth, but low at 3.0 m depth. The microorganisms' phenotypes and genotypes were evaluated, respectively, using carbon-utilization tests and by eubacterial 165 rRNA gene based T-RFLP. Low dominance of the VFA-utilizing bacteria in samples and low concentrations of VFAs in all waste layers suggest that the organic decomposition in this landfill site remained. Gamma-proteobacteria dominated the microbial community at 17.5 m depth. Clostridia were detected at 7.0, 11.5, and 17.5 m depths, suggesting strict anaerobic conditions in these deep layers. The Shannon-Weaver diversity index showed lower values at 3.0 m and 11.5 m depth with a T-RF pattern. The diversity index calculated from the carbon-utilization pattern increased slightly with depth at the landfill site. The landfill-site waste layers are expected to be mutually isolated and to form unique microbial communities depending on the buried wastes' composition, temperature, moisture content, and pressure inside the landfill. (C) 2009, The Society for Biotechnology, Japan. All rights reserved.