Genetic analyses previously implicated the matrix-localized protease ClpP in signaling the stress of protein misfolding in the mitochondrial matrix to activate nuclear-encoded mitochondrial chaperone genes in C. elegans (UPR^mt). Here, we report that haf-1, a gene encoding a mitochondria-localized ATP-binding cassette protein, is required for signaling within the UPR^mt and for coping with misfolded protein stress. Peptide efflux from isolated mitochondria was ATP dependent and required HAF-1 and the protease ClpP. Defective UPR^mt signaling in the haf-1-deleted worms was associated with failure of the bZIP protein, ZC376.7, to localize to nuclei in worms with perturbed mitochondrial protein folding, whereas zc376.7(RNAi) strongly inhibited the UPR^mt. These observations suggest a simple model whereby perturbation of the protein-folding environment in the mitochondrial matrix promotes ClpP-mediated generation of peptides whose haf-1-dependent export from the matrix contributes to UPR^mt signaling across the mitochondrial inner membrane.