SUMMARYThe Cabb S isolate of cauliflower mosaic virus (CaMV) is the only isolate that can be recovered from turnip plants mixedly infected with Cabb S and an infectious variant of Cabb S. Analysis of the progeny resulting from mixed infections of turnip plants was done for a better understanding of this dominance. Cabb S DNA was the dominant DNA recovered from plants after a mixed infection with Cabb S and D/H or W isolates of CaMV, whereas UM130 (derived from Cabb S by the insertion of a short oligonucleotide in open reading frame III) coexisted in turnip plants with D/H, NY8153, CM4-184 and W isolates. To determine whether there are additional sequences responsible for the ability of Cabb S to dominate over UM130, coinoculation experiments were done using chimeric DNAs, obtained by in vivo recombination (IC141, IC143 and VR246) and by in vitro construction (W/CS and CS/W). Chimeras IC141, IC143 and VR246 coexisted in turnip plants with UM130. Chimera CS/W dominated over UM 130, whereas UM 130 dominated over W/CS. The location of Cabb S sequences in these chimeras suggests that more than one region is responsible for the competitive advantage of Cabb S DNA. A different modification of Cabb S DNA in open reading frame III also destroyed the ability of the DNA to dominate over the W isolate. In DNA from virions obtained after co-infection with U M 130 and CS/W, more molecules were recovered with U M 130-specific sequences at position 1364 than with those at position 2040. This and similar results with progeny from W/CS and UM130 co-infection suggest that genetic exchanges had occurred. However, exchanges were not detected in progeny from other mixed infections. Thus the dominance of the Cabb S isolate in mixed infections was due primarily to a better competitive ability of the Cabb S DNA. Plants inoculated first with UM130 and 2 to 8 days later with Cabb S contained only UM130 CaMV DNA, indicating that dominance was unable to overcome cross-protection.