The composition of the editosome, a multi-protein complex that catalyzes uridine insertion and deletion RNA editing to produce mature mitochondrial mRNAs in trypanosomes, was analyzed by mass spectrometry. The editosomes were isolated by column chromatography, glycerol gradient sedimentation, and monoclonal antibody affinity purifications. At least 16 proteins form the catalytic core of the editosome, and additional associated proteins were identified. Analyses of mitochondrial fractions identified several non-editosome proteins and multi-protein complexes. These studies contribute to the functional annotation of T. brucei genome.