The potentially specific importance of the study of heavy ions from galactic cosmic rays for the understanding of radiation protection in manned spaceflight continues to stimulate spaceflight experiments in order to investigate the radiobiological properties of these ions. Chromosome aberrations as an expression of a direct assault on the genome are of particular interest in view of carcinogenesis as the primary radiation risk for man in space. An essential technical ingredient of such spaceflight experiments is the visual nuclear track detector which permits identification of those biological test organisms which have been affected by cosmic heavy ions. We describe such a technique and report on an analysis of the qualitative and quantitative reliability of this identification of particle trajectories in layers of biological test organisms. The incidence of chromosome aberrations in cells of lettuce seeds, Lactuca sativa, exposed during the Kosmos 1887 mission, was determined for seeds hit by cosmic heavy ions. In those seeds the incidence of both single and multiple chromosome aberrations was enhanced.