Silica scale, one of the major problems for geothermal power development, was formed on the copper test plates in geothermal hot water (85 +- 2 degrees C, pH 7.2). Numerous bacterial shaped structures were observed and mixed bacterial population of genomic DNA was stably extracted from the silica scale. The amount of silica scale and extracted DNA increased exponentially with the time of incubation in the geothermal hot water. The molecular phylogenetic survey of extremely thermophilic bacterial diversities in silica scale was carried out by using the PCR-mediated small subunit rRNA gene (rDNA) sequencing. The community was composed mainly of three phylogenetic types in domain Bacteria. Cluster I was affiliated with the Aquificales and cluster n was closely related to the genus Thermus. Cluster III was homologus with Gram-positive anaerobic thermophilic bacteria. These bacterial communities may possibly contribute to the rapid aggregation of silica