Coenzyme Q (Q) functions in the electron transport chain of both prokaryotes and eukaryotes. The biosynthesis of Q requires a number of steps involving at least eight Coq polypeptides. Coq5p is required for the C-methyltransferase step in Q biosynthesis. In this study we demonstrate that Coq5p is peripherally associated with the inner mitochondrial membrane on the matrix side. Phenotypic characterization of a collection of coq5 mutant yeast strains indicates that while each of the coq5 mutant strains are rescued by the Saccharomyces cerevisiae COQ5 gene, only the coq5-2 and coq5-5 mutants are rescued by expression of Escherichia coli ubiE, a homolog of COQ5. The coq5-2 and coq5-5 mutants contain mutations within or adjacent to conserved methyltransferase motifs that would be expected to disrupt the catalysis of C-methylation. The steady state levels of the Coq5-2 and Coq5-5 mutant polypeptides are not decreased relative to wild type Coq5p. Two other polypeptides required for Q biosynthesis, Coq3p and Coq4p, are detected in the wild type parent and in the coq5-2 and coq5-5 mutants, but are not detected in the coq5-null mutant, or in the coq5-4 or coq5-3 mutants. The effect of the coq5-4 mutation is similar to a null, since it results in a stop codon at position 93. However, the coq5-3 mutation (G304D) is located just four amino acids away from the C terminus. While C-methyltransferase activity is detectable in mitochondria isolated from this mutant, the steady state level of Coq5p is dramatically decreased. These studies show that at least two functions can be attributed to Coq5p; first, it is required to catalyze the C-methyltransferase step in Q biosynthesis and second, it is involved in stabilizing the Coq3 and Coq4 polypeptides required for Q biosynthesis.