To investigate the mechanisms that ensure the dependency relationships between cell cycle events and to investigate the checkpoints that prevent progression through the cell cycle after DNA damage, we have isolated mutants defective in the checkpoint and feedback control pathways. We report the isolation and characterization of 11 new loci that define distinct classes of mutants defective in one or more of the checkpoint and feedback control pathways. Two mutants, rad26.T12 and rad27.T15, were selected for molecular analysis. The null allele of the rad26 gene (rad26.d) shares the phenotype reported for the "checkpoint rad" mutants rad1, rad3, rad9, rad17, and hus1, which are defective in the radiation checkpoint and in the feedback controls that ensure the order of cell cycle events. The null allele of the rad27 gene (rad27.d) defines a new class of Schizosaccharomyces pombe mutant. The rad27 complementing gene codes for a putative protein kinase that is required for cell cycle arrest after DNA damage but not for the feedback control that links mitosis to the completion of prior DNA synthesis (the same gene has recently been described by Walworth et al. (1993) as chk1). These properties are similar to those of the rad9 gene of Saccharomyces cerevisiae. A comparative analysis of the radiation responses in rad26.d, rad26.T12, and rad27.d cells has revealed the existence of two separable responses to DNA damage controlled by the "checkpoint rad" genes. The first, G2 arrest, is defective in rad27.d and rad26.d but is unaffected in rad26.T12 cells. The second response is not associated with G2 arrest after DNA damage and is defective in rad26.d and rad26.T12 but not rad27.d cells. A study of the radiation sensitivity of these mutants through the cell cycle suggests that this second response is associated with S phase and that the checkpoint rad mutants, in addition to an inability to arrest mitosis after radiation, are defective in an S phase radiation checkpoint.