The time of occurrence of postirradiation death was studied in Chlamydo-monas reinhardi. Death usually occurs at a time when 4 or 8 daughter cells have been produced. The daughters are uninucleate. In the normal division cycle, 4 or 8 nuclei are produced by successive mitoses, before cytoplasmic division begins, at which time a multinucleate cell is rapidly reduced to a colony containing a corresponding number of uninucleate cells. Prior irradiation does not interrupt this series of division events but prevents the uninucleate progeny from undergoing further mitosis. In some cases, death is not preceded by division. Dose fractionation reduces killing but does not preferentially affect predivisional or postdivisional death. The relative frequencies of "early" and "late" death depend on the effective dose. The number of daughter cells formed is equal to the number of nuclei formed. These observations suggest that an irreversible radiation-induced division block is the result of interference with some process occurring only in the uninucleate stage.