The murine cytomegalovirus (MCMV) M97 gene is homologous with both eukaryotic protein kinases and the phosphotransferases of herpesviruses. The gene conserves the domain structure of protein kinases and of the human cytomegalovirus UL97 (phosphotransferase) gene. An M97 transcript of 2.5 kb is present predominantly at late times, and much smaller quantities of the transcript are detected at early times postinfection. Comparison of the DNA sequences of the complete M97 genes from 12 ganciclovir-sensitive and aciclovir-sensitive strains of MCMV showed that the sensitive isolates strongly conserve the sequence of the catalytic domains, but have only moderate conservation of the sequence of the amino-terminal (regulatory) region. MCMV provides a useful model for studying the in vivo function of the phosphotransferase genes of the betatherpesviruses and has potential for use in studies of antiviral resistance.