RECENT advances in near-field(1) and far-field(2,3) fluorescence microscopy have made it possible to image single molecules and measure their emission(3,4) and excitation(5) spectra and fluorescence lifetimes(3,6-8) at room temperature. These studies have revealed spectral shifts(4) and intensity fluctuations(6,7), the origins of which are not clear. Here we show that spontaneous fluctuations in the spectra of immobilized single dye molecules occur on two different timescales : hundreds of milliseconds and lens of seconds, indicating that these fluctuations have two distinct activation energies, In addition, we see photoinduced spectral fluctuations on repealed photoexcitation of single molecules. We suggest that ail of these fluctuations can be understood as transitions between metastable minima in the molecular potential-energy surface.