A cold-sensitive mutant of a haploid strain of Saccharomyces cerevisiae has been isolated by selection for impaired growth at 18 degrees on nonfermentable carbon sources. Growth of the mutant on glucose or galactose at either 28 or 18 degrees is similar to that of the parental strain. The cold-sensitive strain is highly mutable to a cytoplasmic petite when grown at 18 degrees , or when grown at 28 degrees in the presence of 4 mg/ml of chloramphenicol. Cold sensitivity is not observed in the conversion of promitochondria to mitochondria. We conclude that mitochondrial protein synthesis is required for maintaining the stability of the mitochondrial genome.