Trypanosoma brucei mitochondria contain unusual small circular DNAs of unknown function. These are catenated with a long informational DNA sequence containing genes homologous to those found in other mitochondria. Although these genes are transcribed throughout the life cycle, differential production of the mitochondrial respiratory system during the life cycle is accompanied by differential abundance of specific transcripts and differential polyadenylation of mitochondrial gene transcripts. Multiple transcripts occur for most of the mitochondrial genes. Transcripts of the apocytochrome b gene possessing nucleotide sequences at their 5 ends which are not present in mitochondrial DNA are found at stages of the life cycle when the respiratory system is expressed. These results suggest the presence of post-transcriptional mechanisms that regulate the expression of mitochondrial genes during the life cycle of T. brucei.