Low-temperature (1.4 K), single-molecule fluorescence-excitation spectra have been recorded for individual reaction center-lightharvesting 1 complexes from Rhodopseudomonas palustris and the PufX ؊ strain of Rhodobacter sphaeroides. More than 80% of the complexes from Rb. sphaeroides show only broad absorption bands, whereas nearly all of the complexes from Rps. palustris also have a narrow line at the low-energy end of their spectrum. We describe how the presence of this narrow feature indicates the presence of a gap in the electronic structure of the light-harvesting 1 complex from Rps. palustris, which provides strong support for the physical gap that was previously modeled in its x-ray crystal structure.bacterial photosynthesis ͉ light-harvesting complex ͉ single-molecule spectroscopy