The maturation of mRNA was studied for the Leptomonas seymouri mitochondrial A6 gene coding for ATPase subunit 6. RT-PCR showed that the cytochrome b and A6 genes are transcribed to produce a polycistronic RNA. The RT-PCR products, containing the cDNAs of these genes, were cloned and sequenced. Comparison of the sequence for 12 clones revealed four types of transcripts representing partly or completely edited products of A6. Two different variants of completely edited A6 mRNAs were found.