We have investigated RNA recombination among poliovirus genomes by analyzing both intratypic and intertypic recombinant crosses involving the same defined genetic markers. Sequence analysis of the recombinant junctions of 13 nonsibling intertypic recombinants showed that intertypic RNA recombination is not site-specific, nor does it require extensive homology between the recombining parents at the crossover site. To discriminate between breaking-rejoining and copy choice mechanisms of RNA recombination, we have inhibited the replication of the recombining parents independently and found opposite effects on the frequency of genetic recombination in intratypic crosses. The results strongly support a copy choice mechanism for RNA recombination, in which the viral RNA polymerase switches templates during negative strand synthesis.