Paramyxoviruses cotranscriptionally edit their P gene mRNA by the programmed insertion of G residues into a short G run contained within a larger purine run, via pseudo-templated transcription. The templates for paramyxovirus transcription are genome nucleocapsids in which each nucleoprotein subunit is associated with 6 nt, and only genomes whose lengths are multiples of 6 are found naturally or are replicated efficiently in transfected cell systems. We have examined the effect of varying total genome length on the frequency and number of insertions into the mRNA editing site in a transfected cell system, using constructs that generate mini-genome analogues. We found that, as long as the purine run sequence and the region immediately upstream were unaltered, editing occurred during mRNA synthesis independent of the precise length of the minigenome. However, when mini-genome constructs whose lengths were not multiples of 6 were used, insertions (or deletions) occurred during antigenome synthesis within the purine run, which strikingly restored the hexamer length. Genome length correction due to changes in the antigenome purine run length occurred only when the mini-genome was not a multiple of 6, and these changes were only poorly affected by mutations in the mRNA editing site and the region immediately upstream. Our results suggest that the mRNA editing site is a natural hotspot for viral polymerase slippage during genome replication, and that this site serves the dual and complementary function of maintaining hexamer genome length. The unusual requirement of paramyxoviruses for genomes of precise hexamer length may have evolved to maintain genome stability against insertions in the mRNA editing site during replication.