The controlled manipulation and switching of single atoms and molecules raise the prospect of ultra-high-density data storage, Switching by motion of a single atom has been reported(1), and techniques of single-molecule optical detection and spectroscopy(2) in the condensed phase have been refined to a degree that allows the modification of the absorption properties of a single chromophore(3). Light-induced jumps in single-molecule excitation frequencies have been reported(3-5), but in none of these cases could the process be controlled : the jumps varied from molecule to molecule, they were interrupted by spontaneous jumps, and the new excitation frequencies could not be identified unambiguously, Here we report light-induced reversible frequency jumps of single molecules of the aromatic hydrocarbon terrylene embedded in a particular site of a p-terphenyl host crystal(6) at temperatures of around 2 K. The changes in absorption frequency for different terrylene molecules were identical (within 0.5%) for all samples studied, Thus we were able to switch single-molecule absorption lines in a controlled way between well-defined frequency positions.