Mitochondrial genomes (mitochondrial DNA, mtDNA) encode essential oxidative phosphorylation (OXPHOS) components. Because hundreds of mtDNAs exist per cell, a deletion in a single mtDNA has little impact. However, if the deletion genome is enriched, OXPHOS declines, resulting in cellular dysfunction. For example, Kearns-Sayre syndrome is caused by a single heteroplasmic mtDNA deletion. More broadly, mtDNA deletion accumulation has been observed in individual muscle cells1and dopaminergic neurons2during ageing. It is unclear how mtDNA deletions are tolerated or how they are propagated in somatic cells. One mechanism by which cells respond to OXPHOS dysfunction is by activating the mitochondrial unfolded protein response (UPRmt), a transcriptional response mediated by the transcription factor ATFS-1 that promotes the recovery and regeneration of defective mitochondria3,4. Here we investigate the role of ATFS-1 in the maintenance and propagation of a deleterious mtDNA in a heteroplasmicCaenorhabditis elegansstrain that stably expresses wild-type mtDNA and mtDNA with a 3.1-kilobase deletion (ΔmtDNA) lacking four essential genes5. The heteroplasmic strain, which has 60%ΔmtDNA, displays modest mitochondrial dysfunction and constitutive UPRmt activation. ATFS-1 impairment reduced theΔmtDNA nearly tenfold, decreasing the total percentage to 7%. We propose that in the context of mtDNA heteroplasmy, UPRmt activation caused by OXPHOS defects propagates or maintains the deleterious mtDNA in an attempt to recover OXPHOS activity by promoting mitochondrial biogenesis and dynamics.