A DNA-dependent RNA polymerase has been purified to homogeneity from bacteriophage N4 virions. Sodium dodecyl sulfate-8 M urea polyacrylamide gel electrophoresis of the enzyme revealed a single polypeptide with a molecular weight of 350,000. The hydrodynamic properties of the enzyme have been determined to be 9.5 S for the sedimentation coefficient and 84 A for the Stokes radius. These two parameters indicate a native molecular weight of 320,000. Enzyme activity is dependent on the presence of Mg2+, the four ribonucleoside triphosphates, and denatured N4 DNA. Under these conditions, initiation of RNA synthesis occurs exclusively with pppG. The enzyme is inhibited by monovalent salts and is resistant to the drugs rifampicin and streptolydigin. The protein is present in 1 to 2 copies per virion; its presence provides an explanation for the independence of N4 early RNA synthesis on the activity of the host RNA polymerase.