Yeast mitochondria were fractionated into inner membrane, outer membrane, matrix, and intermembrane space. Identity and purity of each fraction were monitored by enzyme assays, dodecyl sulfate-polyacrylamide gel electrophoresis, and immunological detection of characteristic mitochondrial polypeptides. Cytochrome b2 and cytochrome c peroxidase were found to be components of the intermembrane space. The most reliable marker of the outer membrane was a major 29,000-dalton polypeptide component. The availability of submitochondrial fractions provides a basis for studying import of precursor polypeptides into isolated yeast mitochondria.