RNA editing is the specific posttranscriptional insertion/deletion of U residues within trypanosomatid mitochondrial transcripts. We used a novel cDNA cloning scheme and analyzed partially edited COIII and CYb RNAs. Our major unanticipated findings are: First, editing appears strikingly indiscriminate within editing domains, inserting and deleting variable numbers of U residues at all sites, both those requiring editing and those that do not. Second, this incomplete editing occurs in precise domains, not in adjacent regions unedited in mature mRNA. Third, editing appears to commence prior to polyadenylation. We propose that within a region being edited, there are cycles of indiscriminate cleavage, U addition or deletion, and religation; sites that become correctly edited would be protected from further modification by duplexing with short RNAs complementary to the edited sequence.