Ribulose-1,5-bisphosphate carboxylase/oxygenase of chloroplasts contains eight large and eight small subunits. The small subunit is encoded by nuclear DNA, synthesized in the cytoplasm, and imported into chloroplasts. The large subunit is encoded by chloroplast DNA and synthesized within chloroplasts. We show in this communication that the large subunit of Chlamydomonas chloroplasts could be efficiently imported into isolated yeast mitochondria if it was attached to the presequence of a protein transported into the yeast mitochondrial matrix. Thus, synthesis of the large subunit within chloroplasts does not reflect the inability of this subunit to cross membranes. The same mitochondrial presequence could also transport the nuclear-encoded small subunit into yeast mitochondria. However, when the two types of subunits were coimported into mitochondria, they did not assemble with each other inside the heterologous organelle.