A computer-controlled microdensitometer has been designed and built mainly for use in X-ray crystallography. The instrument is a hybrid of two earlier microdensitometers and combines accurate positioning with fast measurement. It incorporates a stage mechanically driven by stepping motors. The source of light is a high-resolution cathode ray tube which is imaged on to the film plane, the optical densities being measured by photomultipliers. The instrument is used routinely to scan oscillation photographs of biological macromolecules.