The genome of murine cytomegalovirus, extracted from extracellular virions, is a linear double-stranded DNA molecule ca. 240 kilobase pairs long. In our initial cloning of subgenomic fragments of the murine cytomegalovirus genome, we obtained a HindIII clone which contained fused HindIII-terminal fragments. By hybridizing this cloned DNA fragment to infected-cell DNA, we identified an intracellular restriction fragment which was the length of the sum of the two authentic termini. This fusion fragment was not present in virion DNA but could be detected as early as 2 h postinfection and reached its highest level shortly after the onset of DNA replication at 16 h postinfection. The prereplicative increase of fused ends was not inhibited by a level of phosphonoacetic acid which effectively shut off viral DNA synthesis, nor was the early conversion from free to fused ends prevented by inhibitors of protein or RNA synthesis. The results are consistent with the fused state of viral DNA being a replicative intermediate and precursor to DNA synthesis.