Seven temperature-sensitive mutants of vaccinia virus have been isolated after preselection for virus resistant to phosphonoacetic acid (PAA). In all seven mutants, the PAA-resistant (PAAr) and ts lesions represent separate mutations. In one mutant, NG26, the PAAr (NG26-PAAr) and ts (NG26-ts) mutations are very closely linked. Both NG26-ts and NG26-PAAr map in the HindIII E DNA fragment. NG26 has a DNA-negative phenotype at 40 degrees. NG26-ts is in the same complementation group as ts42, another DNA-negative mutant which maps in the HindIII E DNA fragment (R. C. Condit, A. Motyczka, and G. Spizz, Virology 128, 000-000, 1983). The order of the mutations is (NG26-ts)-(NG26-PAAr)-ts42. The virus-coded DNA polymerase has been partially purified from wt- and NG26-infected cells. The DNA polymerase encoded by NG26 is temperature sensitive and PAA resistant in vitro as compared to the wt enzyme.