The inner membranes of mitochondria contain a family of transport proteins of related sequence and structure. The DNA sequence of the genome of Saccharomyces cerevisiae encodes at least 35 members of this family. Three of them can be recognised as known isoforms of the ADP-ATP translocase and two others as the phosphate and citrate carriers. The transport functions of the remainder cannot be identified with certainty. One of them, encoded on yeast chromosome xii, shows a fairly close sequence relationship to the known sequence of the bovine mitochondrial oxoglutarate-malate carrier. The yeast protein has been obtained by over-expression in Escherichia coli, reconstituted into phospholipid vesicles and shown to have transport properties characteristic of the mitochondrial carrier for dicarboxylate ions, such as malate, and also phosphate, previously biochemically characterised, but not sequenced, from both mammalian and yeast mitochondria. This is the first example of the biochemical identification of an unknown membrane protein encoded in the yeast genome since the completion of the genomic sequence.