Coinfection of MA-104 cells with serotype 1 (Wa strain) and serotype 3 (P strain) human rotaviruses resulted in progeny viruses that were phenotypically mixed in their major outer-shell structural-protein VP7 and in the genome segment that encodes this protein (segment 9). Segments from the Wa virus predominated in these progeny whether they were of parental or reassortant genotype. Neutralization with monoclonal antibodies specific for VP7 proteins of the coinfecting viruses caused an alteration in genomic distribution favoring the strain heterologous to the neutralizing monoclonal antibody. Because the percentage of change in distribution of segment 9 was similar to that of the other combined segments, there appeared to be no greater association between VP7 and segment 9 than with other segments of the homologous virus during encapsulation. From these results, it was calculated that the progeny of coinfection with P segment 9 were 77.4% mosaic structures and 14.8% pseudotypes; progeny of coinfection with Wa segment 9 were 40.2% mosaic structures and 1.3% pseudotypes. Similar determinations were made for the reassortant progeny alone.