At 45 C, in a temperature-sensitive initiation mutant (TsB134) of Bacillus subtilis 168 Thy- tryp-, growing in a glucose-arginine minimal medium, chromosome completion occurred over a period of 80 to 90 min, after which there was no further nuclear division. Normal symmetrical cell divisions continued for a generation afterwards, so that nuclei were segregated into separate cells. During this period asymmetric divisions started to occur. Septa appeared at 25 to 30% from one end of the cell, giving a small anucleate cell and a larger nucleate cell. During inhibition of deoxyribonucleic acid (DNA) synthesis by thymine starvation under the restrictive conditions, asymmetrical division also occurred until there was approximately one nucleus per cell (about one generation time). Asymmetric division, giving anucleate cells, then occurred. Similar results were obtained when DNA synthesis was inhibited by nalidixic acid. After 3 h at 45 C, the rate of anucleate cell production in the presence and absence of thymine was constant at one division per 85 min per chromosome terminus present when DNA synthesis stopped. In the absence of DNA synthesis (during thymine starvation) at 35 C, growth in cell length was linear (i.e., the rate was constant), but at 45 C during thymine starvation the rate gradually increased by more than twofold. It is suggested that this was due to the establishment of new sites of growth associated with anucleate cell production. In the presence of thymine at 45 C, the rate of length extension increased by more than fourfold, which it is suggested was caused by the appearance of new growth zones as a result of chromosome termination and a contribution associated with anucleate cell production. If the mutant was incubated at 45 C for 90 min, both in the presence and absence of thymine, then anucleate cell formation could continue on restoration to 35 C in the absence of thymine...