The gene for cytochrome oxidase subunit III (COIII) is not detected in T. brucei by nucleotide sequence analysis or by cross-hybridization studies. We have identified the COIII transcript in the T. brucei mitochondrion by RNA and cDNA sequencing. Its nucleotide and predicted amino acid sequences are very similar to those from the COIII genes in related species. A mitochondrial DNA sequence that matches the COIII transcript, except for the absence of numerous thymidines, is located upstream of the apocytochrome b gene, the position of the COIII gene in related species. We conclude that this sequence is the COIII gene and that over 50% of the T. brucei COIII transcript sequence is created by RNA editing.