The primary structures of iso-1-cytochrome c and iso-2-cytochrome c in the yeast Saccharomyces cerevisiae are determined by the genes CYC1 and CYC7, respectively. The CYC1 locus was previously shown to be on the right arm of chromosome X, and the CYC7 locus is shown in this investigation to be on the left arm of chromosome V closely linked to the min1 and mak10 markers. The CYC7 locus appears to be composed of a structural region and a regulatory region. Mutations in the structural region can cause a deficiency or alteration of iso-2-cytochrome c, whereas mutations in the regulatory region can cause increases in the amount of iso-2-cytochrome c. Single-site gene conversion, occurring at a relatively high frequency of approximately 4%, caused intragenic recombination of a mutational site in the structural region and a mutational site in the regulatory region, enabling us to suggest the order of the sites in relationship to other markers on the chromosome.