The archaebacteria constitute a group of prokaryotes with an intermediate phylogenetic position between eukaryotes and eubacteria. The study of their DNA polymerases may provide valuable information about putative evolutionary relationships between prokaryotic and eukaryotic DNA polymerases. As a first step towards this goal, we have purified to near homogeneity a DNA polymerase from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius. This enzyme is a monomeric protein of 100 kDa which can catalyze DNA synthesis using either activated calf thymus DNA or oligonucleotide-primed single-stranded DNA as a template. The activity is optimal at 70 degrees C and the enzyme is thermostable up to 80 degrees C; however, it can still polymerize up to 200 nucleotides at 100 degrees C. These remarkable thermophilic properties and thermostability permit examination of the mechanism of DNA synthesis under conditions of decreased stability of the DNA helix. Furthermore, these properties make S. acidocaldarius DNA polymerase a very efficient enzyme to be used in DNA amplification by the recently developed polymerase chain reaction method (PCR) as well as in the Sanger DNA sequencing technique.