Genetic analysis of a large number of radiation-sensitive mutants of S. pombe, isolated in different laboratories, showed that these isolates represent 22 non-allelic loci. The mutants were shown to fall into three distinct classes concerning response to UV and ionizing radiation, including two mutants which are primarily sensitive to ionizing radiation but not to UV. Single-gene mutants were crossed to obtain supersensitive double mutants. Such double mutants showed a marked increase in sensitivty to a variety of inactivating agents as compared to the parental strains. The isolation of three classes of radiation-sensitive mutants and the construction of double mutants implies the presence of multiple pathways in S. pombe for repair of radiation-induced damage. The bearing of these data on cellular repair mechanisms in eukaryotes is discussed.