A DNA-dependent DNA polymerase was obtained in homogenous form from the thermoacidophilic archaebacterium Sulfolobus solfataricus. The enzyme, purified 706-fold, has a molecular mass of about 110000 daltons as determined by gel filtration and by glycerol gradient centrifugation. It requires Mg++ for its activity and has a pH optimum of 7.7. The activity is sharply dependent on the ionic strength. The enzyme is thermostable; its properties and activity requirements were characterized. The features of this enzyme are compared to those of other DNA polymerases isolated either from prokaryotes or eukaryotes.