Mutations in two genes (cdc8 and cdc21) required for nuclear and mitochondrial DNA synthesis in Saccharomyces cerevisiae result in a 6- to 11-fold increase in the rate of mitotic segregation of petites at the permissive temperature. The defect in DNA replication and the increased rate of petite production result from the same mutation since the two phenotypes cosegregate and corevert. Most of the petites isolated from strains carrying mutations in cdc8 and cdc21 contain mtDNA. Therefore, the petites do not result simply from an underreplication of mitochondrial DNA. The mutation rates for nuclear and mitochondrial genes are the same in cdc8, cdc21 and their wild-type parent. Therefore the petites are unlikely to result from an increase in the rate of base pair substitution.