Nascent RNA, synthesized by calf thymus RNA polymerase II on restriction endonuclease BamHI linearized hamster papovavirus (HaPV) DNA, was rehybridized to the template strand under conditions allowing transcription R-loop formation. Hybrids, visualized by electron microscopy, were plotted and mapped according to the physical map of HaPV. Two predominant regions of transcription could be localized at 0.10--0.40 and 0.50--0.82 m.u., respectively. For the start sites of transcription at map positions 0.67 and 0.75, respectively, on the HaPV genome a transcription in opposite direction were estimated. This genome region harbours the putative origin of replication of HaPV DNA. These results suggest a distinct relatedness of HaPV to the polyomavirus group.