Photosystem I (PS I) reaction centers are nanometer-size robust supramolecular structures that can be isolated and purified from green plants. Using the technique of Kelvin force probe microscopy, we report here the first measurement of exogenous photovoltages generated from single PS I reaction centers in a heterostructure composed of PS I, organosulfur molecules, and atomically flat gold. Illumination of the reaction centers was achieved with a diode laser at lambda = 670 nm. Data sets consisting of 22 individual PS Is measured entirely under laser illumination, 12 PS Is measured entirely in darkness, and four PS Is in which the light-dark transition occurred in midscan of a single PS I were obtained. The average values of the light minus dark voltages relative to the substrate for the four PS Is were -1.13 +/- 0.14 and -1.20 +/- 0.19 V at diametrical peripheries＇ and -0.97 +/- 0.04 V at the center. Under illumination, the potentials of the central region of the PS Is were typically more positive than the periphery by 6-9 kT, where kT is the Boltzmann energy at room temperature. These energies suggest a possible mechanism whereby negatively charged ferredoxin, the soluble electron carrier from PS I to the Calvin-Benson cycle, is anchored and positioned at the reducing end of PS I for electron transfer. The results are placed in context with the prior experimental literature on the structure of the reducing end of PS I.