A number of cytosines are altered to be recognized as uridines in transcripts of the NADH-dehydrogenase subunit 3 (nad3) gene in the mitochondria of the higher plant Petunia hybrida. Here we show that the extent of editing for three of the edit sites, all of which change the encoded amino acid, varies between different Petunia lines. Genetic analysis indicates that a single nuclear gene is responsible for this variation. Interestingly, according to RNA blot hybridization analysis, RNA editing extent and transcript abundance are correlated. This observation is consistent with the hypothesis that RNA editing is a post-transcriptional event.