A DNA polymerase has been isolated from the thermophylic bacteria Thermus aquaticus YT-1. The six-step purification procedure resulted in an electrophoretically homogeneous enzyme preparation with molecular weight of about 62 000. The enzyme does not contain contaminant exo- and endonuclease activities and has a temperature optimum on the DNA templates at 70 degress and that on RNA matrices at 50 degrees. The maximal activity of the enzyme requires the presence of bivalent cations (magnesium or manganese) 0,1--0,2 M KCl or NaCl, all deoxynucleoside triphosphates and template in the incubation mixture. The enzyme is active when "activated" DNA, poly(dA)-poly(dT), poly(dA)-oligo(dT)10 and poly(rA)-oligo(dT)10 are used as templates and is inactive on the native and denaturated DNAs as well as on the native molecules of RNA and poly(rC)-oligo(dG)12--180.