A pea cDNA clone, PHSP1, encoding a member of the HSP70 gene family has been isolated. DNA sequence analysis indicates that the protein encoded by PHSP1 is a homologue of the mitochondrial HSP70 proteins, SSP1 from Schizosaccharomyces pombe and SSC1 from S. cerevisiae. It contains an amino-terminal extension of 50 amino acids, rich in basic and hydroxyl amino acids, similar to other plant mitochondrial leader sequences. Western blot analysis indicates that the PHSP1 protein is associated only with mitochondria and not with any other sub-cellular organelle or cytoplasm. Further confirmation of its location within mitochondria was obtained from in vitro protein translocation experiments into purified Pisum sativum mitochondria. It was observed that the precursor protein was efficiently imported and that it is processed to produce a protein with an Mr of the anticipated size of the mature protein. Results are discussed with respect to the structure and function of the mitochondrial HSP70 protein.