We have previously isolated the yeast nuclear gene OXA1 and showed that Oxa1p is required for the formation of the cytochrome c oxidase and ATP synthase complexes. We have expressed Oxa1p in E. coli and shown that it is toxic and rapidly degraded. Nevertheless, a truncated protein was successfully expressed and antibodies have been raised against this truncated protein. These antibodies recognise a protein in mitochondrially enriched fractions. In vitro mitochondrial import experiments demonstrate that the import of Oxa1p is accompanied by the cleavage of a long pre-sequence. Osmotic swelling and alkaline carbonate extraction show that Oxa1p is an integral membrane protein located in the inner membrane of mitochondria. The relationships between the sub-mitochondrial location and the function of Oxa1p are discussed.