Experimental evidence exists that RNA viruses replicate with extremely high mutation rates that result in significant genetic diversity. The diverse nature of viral populations allows rapid adaptation to dynamic environments, and evolution of resistances to vaccines as well as antiviral substances. For DNA viruses that replicate at much greater fidelities, as yet, neither diverse structures in the population nor their responses to increased mutation rates have been sufficiently described. By using the example of DNA bacteriophage T7, we describe the identification of virus-specific DNA polymerase variants with decreased replication fidelities, and their impact on the efficiency of the viral infection cycle.