Prototype strain MG409 (arg11-1) is a severe arginine bradytroph with greatly reduced ornithine and arginine pools, although all known enzymes required for arginine biosynthesis are functional. To identify the function required for normal arginine production impaired in MG409, we have cloned, sequenced, and performed a first molecular characterization of ARG11. We show that the ARG11 open reading frame encodes a putative 292-residue protein with a predicted molecular mass of 31.5 kDa. Sequence similarities, a tripartite organization, and six potential hydrophobic transmembrane spans suggest that Arg11p belongs to the mitochondrial integral inner membrane carrier family. We have used immuno-Western blotting and hemagglutinin epitope-tagged derivatives of Arg11p, Arg8p (a mitochondrial matrix marker), and Arg3p (a cytosolic marker) to demonstrate that Arg11p is confined to the mitochondria and behaves like an integral membrane protein. A deletion created in ARG11 causes the same arginine-leaky behavior as the original arg11-1 mutation, which yields a premature stop codon at residue 266. Arg11p thus appears to fulfill a partially redundant function requiring its 27 carboxyl-terminal amino acids. As a working hypothesis, we propose that Arg11p participates in the export of matrix-made ornithine into the cytosol.