Rous sarcoma virus (RSV)-transformed Chinese hamster fibroblasts, containing approximately ten copies of the DNA domain comprising a single provirus and its flanking cellular sequences, were arrested in metaphase, and the chromosomes were fractionated by size in a sucrose gradient. The resolution of polymorphic ribosomal genes, the dihydrofolate reductase gene, and the c-src gene demonstrated that the gradient can distinguish between small, medium, and large chromosomes. The same DNA domain carrying the RSV provirus was found to be associated with chromosomes of all three size classes. Polymorphic copies of the domain in small and large chromosomes could be distinguished from those in medium-sized chromosomes because of the polymorphism in the Xhol and EcoRl sites on 5’ and 3’ adjacent cellular sequences, respectively. The presence of the same provirus domain on different chromosomes, together with the karyological data showing trisomies in the same chromosome size classes, suggest that the provirus domain, possibly the entire replicon, was duplicated and transferred to different nonhomologous chromosomes, and the transfer was followed by duplication of the target chromosomes. The possibility that proviral long terminal repeats might be involved in replicon transfer is discus