A study has been made of the intracellular changes occurring during thymineless death in Bacillus subtilis 2337. The kinetics of death were paralleled by the rate of breakdown of DNA. During thymineless death single-strand breaks accumulated within DNA, breakdown of approximately 13% of the DNA to acid-soluble material occurred, deoxyribonuclease levels rose sharply, and yet double-strand breaks did not occur in the DNA. On restoration of thymine, however, double-strand breaks accumulated, though this could be prevented by the specific inhibition of DNA preplication on restoration of thymine. The results probably indicate that during thymineless death, single-strand gaps accumulate within the DNA of cells. On restoration of thymine both DNA replicaion and repair of gaps are simultaneously initiated, and when a replication fork reaches a gap before it is repaired, double-strand breakage of the DNA occurs. The possible relevance of these events to the lethality of the cells is discussed.