We find that two Chlamydomonas outer arm dynein assembly loci, oda6 and oda9, are located on the left arm of linkage group XII, in the vicinity of the previously mapped locus for a 70,000 Mr dynein intermediate chain protein. Restriction fragment length polymorphism mapping indicates that this dynein gene is very closely linked to the oda6 locus. A cDNA clone encoding the 70,000 Mr protein was isolated, sequenced, and used to select genomic clones spanning the corresponding locus from both wild-type and oda6 libraries. When wild-type clones were introduced into cells containing an oda6 allele, the mutant phenotype was rescued, while no rescue was observed after transformation with oda6 clones. Genetic analysis further revealed that newly introduced gene copies were responsible for the rescued phenotype and thus confirms that ODA6 encodes the 70,000 Mr dynein intermediate chain protein. The inability of oda6 mutants to assemble any major outer arm dynein subunits shows that this protein is essential for assembly of stable outer dynein arms. This is the first use of transformation with a wild-type gene to identify the product of a Chlamydomonas mutant.