The nuclear gene MRP10 of Saccharomyces cerevisiae was cloned by complementation of a respiratory deficient mutant N518/L1. This mutant is defective in mitochondrial translation and shows a tendency to accumulate deletions in mitochondrial DNA (rho-). Analysis revealed Mrp10p to be a component of the 37 S subunit of the mitochondrial ribosomes. Disruption of MRP10 in a haploid strain of yeast elicits a phenotype identical to that of the original mutant. The respiratory defect of the null mutant is rescued by re-introducing the MRP10 gene in a wild-type mitochondrial DNA background. These results indicate that Mrp10p belongs to the class of yeast mitochondrial ribosomal proteins that are essential for translation. Searches of current databases failed to reveal any homologs among known bacterial or eucaryotic cytoplasmic ribosomal proteins. Some sequence similarity, however, was detected between Mrp10p and Yml37p, previously identified as a component of the yeast mitochondrial 50 S ribosomal subunit.