The physical map locations of 62 temperature-sensitive mutations of vaccinia virus WR have been determined by marker rescue experiments, using cloned HindIII fragments of wild-type DNA. Since vaccinia virus DNA is not infectious, marker rescue was performed by infecting monolayers of cells at the nonpermissive temperature with a low multiplicity of the mutant to be rescued and transfecting with calcium phosphate-precipitated recombinant DNA. Wild-type recombinants were measured by using either a direct plaque assay technique or a two-step procedure in which the final yield of virus from the transfected cells was assayed at the permissive and nonpermissive temperatures. Mutants that had been previously assigned to the same complementation-recombination group were rescued by the same HindIII fragment, with the exception of three mutants in one group that were rescued by either one of two adjacent fragments. A comparison between the genetic linkage map of the temperature-sensitive mutations in 30 mutants with their physical locations demonstrated that not only was the order of the genetic map correct but also recombination frequencies generally reflected actual physical distances.