Summary: Can localized electric fields drive the complete unfolding of a protein molecule? Protein unfolding prior to its translocation through a nanopore constriction is an important step in protein transport across biological membranes and also an important step in nanopore-based protein sequencing. We studied here the electric-field–driven translocation behavior of a model protein (cyt c ) through nanopores of diameters ranging from 1.5 to 5.5 nm. These single-molecule measurements show that electric fields at the nanopore constriction can select both partially and fully unfolded protein conformations. Zero-field free energy gaps between these conformations, found using a simple thermodynamic model, are in remarkable agreement with previously reported studies of cyt c unfolding energetics. Datasets used to make figures have been deposited in Figshare ().