To investigate the association between incorporation of 1-beta-D-arabinofuranosylcytosine (ara-C) into herpes simplex virus type 1 (HSV-1) DNA and mutagenesis at selected genetic loci, we grew virus in the presence or absence of ara-C and compared the mutation frequencies. Both the forward mutation frequency of HSV-1 (KOS) to the tk- phenotype and the reversion frequency of a temperature-sensitive mutant to the non-temperature-sensitive (ts+) phenotype were significantly increased following growth in ara-C. When the same viruses were grown in an inhibitor of the HSV-1 DNA polymerase that is not incorporated into DNA, no significant increase in the frequency of tk- or ts+ particles was observed. Furthermore, HSV-1 araAr, a mutant resistant to ara-C on the basis of reduced incorporation of the analog into viral DNA, did not demonstrate an increase in the number of tk- particles following growth in ara-C. These results suggest that mutagenesis of HSV-1 following growth in ara-C correlates with incorporation of the nucleoside analog into viral DNA.