The solution to the riddle of how a protein folds is encoded in the conformational energy landscape for the constituent polypeptide, Employing fluorescence energy transfer kinetics, we have mapped the S. cerevisiae iso-1 cytochrome c landscape by monitoring the distance between a C-terminal fluorophore and the heme during folding. Within 1 ms after denaturant dilution to native conditions, unfolded protein molecules have evolved into two distinct and rapidly equilibrating populations: a collection of collapsed structures with an average fluorophore-heme distance ((r) over bar) of 27 Angstrom and a roughly equal population of extended polypeptides with (r) over bar > 50 Angstrom. Molecules with the native fold appear on a time scale regulated by heme ligation events (similar to300 ms, pH 7). The experimentally derived landscape for folding has a narrow central funnel with a flat upper rim on which collapsed and extended polypepticles interchange rapidly in a search for the native structure.