The mitochondrial unfolded protein response (UPRmt) is attenuated in animals with reduced activity of clpp-1, which encodes a mitochondrial matrix protease homologous to bacterial ClpP. clpp-1 RNAi results in disruption of the UBL-5/DVE-1 complex and interferes with expression of the ubl-5 and dve-1 genes. Peptide efflux from isolated mitochondria was ATP dependent and required HAF-1 and the protease ClpP.