High-LET particle hits in embryos ofZea mays(corn) seeds, flown as part of the ASTP-Biostack III, were determined via plastic nuclear track detectors. Based on etched particle-track measurements, 41 embryos were hit in seed layer 1 which contained 80 seeds, and 49 hits occurred in layer 2 which contained 79 seeds. The mean LET value and range of atomic numbers of recorded hits is, respectively, 210 ± 57 keV/μm and 9 ≲Z≲ 26. Detailed analysis of one particular seed showing marked growth anomalies revealed two hits in the central region of the embryo. These two hits had LET values in the region of 100–150 keV/μm, and Z ≳ 20.