To clarify the relationship between thymineless death and thymineless mutagenesis, the induction of arginine revertants of Escherichia coli TAU-bar by thymine starvation was examined in physiological terms. Induced revertants were detectable both on minimal medium lacking arginine and minimal medium supplemented with 1 mug of arginine per ml. Substantial thymineless mutagenesis occurred during the period before the onset of thymineless death. Mutagenesis and loss of viability were observed upon incubation in medium lacking thymine and arginine, and both were inhibited upon incubation in medium lacking thymine and uracil. Mutagenesis also occurred during thymine starvation at 25 C, where there was relatively little loss of viability. At 37 C thymineless mutagenesis did not require complete thymine starvation, and the induction of revertants appeared to be initiated at the same suboptimal thymine concentration at which lethality was first detectable. Mutagenesis was found not to occur preferentially at the growing point of deoxyribonucleic acid replication. These results suggest that thymineless mutagenesis does not involve simply errors in base pairing due to the absence of thymine. The data also suggest that the induction of mutations and thymineless death are due to the same primary event but that mutagenesis is the more sensitive response.