A DNA-polymerase from the external thermophylic bacteria Thermus ruber has been isolated. A six-step purification procedure resulted in an electrophoretically homogeneous enzyme preparation with m. w. of 70 000. The isolated enzyme is thermostable and has a temperature optimum on DNA templates at 70 degrees and that on RNA templates at 50 degrees. The enzyme does not contain contaminant endo- and exonuclease activities. The maximal activity of the enzyme requires the presence of template, four deoxyribonucleoside triphosphates, monovalent and divalent cations in the incubation mixture. The catalytic activity of the enzyme on various templates of DNA- and RNA-types has been investigated. A comparative physico-chemical study of this DNA-polymerase and an analogous enzyme isolated earlier from the external thermophylic bacteria Thermus aquaticus YT-1 has been carried out.