The photophysical properties of terrylenediimide (TDI) make this novel fluorophore an ideal candidate for single-molecule studies in solid matrices using various optical techniques. By employing frequency selective high-resolution laser spectroscopy at liquid helium temperatures, we could isolate the fluorescence excitation lines of single TDI molecules. Additionally, vibrational modes of the molecule were examined and the population and depopulation rates of the triplet state determined. At temperatures between 100 K and room temperature single TDI molecules were imaged by confocal fluorescence microscopy. In these investigations one-step irreversible bleaching and ''on-off'' transitions of the fluorescence were observed.