According to the endosymbiont hypothesis, mitochondria are descended from ancient aerobic bacteria that were engulfed by protoeukaryotic cells. Experiments described here show that a synthetic peptide corresponding to a yeast mitochondrial targeting sequence can be imported into Paracoccus denitrificans, a soil bacterium thought to be closely related to the protomitochondrion. The import is very similar to that observed with isolated yeast mitochondria. The results suggest that the protomitochondrion may have been inherently able to translocate mitochondrial presequences. This ability may partly explain the development of the protein import process during the evolution of the mitochondrion.