Low temperature two‐photon fluorescence excitation microscopy was used to obtain fluorescence spectra of single CdSe colloidal nanocrystals or quantum dots. The fluorescence spectra reveal large, 0.02 eV shifts of the peak emission wavelength on times of the order of 100 s were also observed in some instances. We attribute this behavior to excited state coupling to photoinduced changes at the nanocrystal surface. ©1996 American Institute of Physics.