The ribulosebisphosphate carboxylase/oxygenase-defective Chlamydomonas mutant, 10-6C, was the first mutant to be physically defined in chloroplast DNA. In this report, a photosynthesis-competent revertant of the 10-6C mutant has been found to result from true reversion within the chloroplast large-subunit gene. This result supports the original assignment of the 10-6C mutation within the large-subunit gene.