In a previous study using reassortant viruses, bovine rotavirus B223 VP7 protein enhanced the neutralization titers of some VP4 specific cross-reactive monoclonal antibodies (MAbs) (Xu and Woode, Virology, 1993). In this report, the influence of the B223 VP7 gene on the growth of reassortants was studied. The growth curves of three B223/69M reassortants with or without the B223 VP7 gene were compared. Reassortant M8, which has B223 VP7 on the genetic background of 69M, replicated as fast and to a similar titer as B223 and better than 69M. In contrast, the growth of reassortants B9, which has 69M VP7 on the B223 background, and of M46, which has B223 VP4 and VP6 on the 69M background, were poorer than B223. As B223 VP7 appeared to provide replication advantages, B223 was cultured as co-infections with each of the following viruses: Wa, 69M, H-2, SA11-4F, and NCDV followed by 20 subpassages. For co-infections with Wa, 69M, H-2, and SA11-4F, B223 VP7 gene was selected, in contrast to the B223/NCDV co-infection, when NCDV VP7 gene was selected. Other genes were also selected non-randomly, but varied among the five co-infection pairs. These data suggest in particular that B223 or NCDV VP7 protein can provide replication advantages to reassortant viruses, possibly by modifying VP4 to make it adsorb more efficiently to the cell receptors.