Ultraviolet (UV) resistance patterns of sporulating Bacillus cereus were determined. Six different categories of UV resistance were discernible as the organism progressed from exponential growth to the free, mature spore. Light microscope observations allowed the assignment of certain sporulation stages to specific UV resistance categories. Marked changes in UV resistance were associated with stages III, mid-IV, and post-IV of sporulation. Dipicolinic acid was shown to sensitize forespores to UV radiation. Mechanisms invoked to explain the different UV categories are discussed.