In a thymine-deprived culture, the mutant cells (deficient in dTDP-glucose pyrophosphorylase activity and named Ter-15) lose viability at a faster rate, form longer filaments for the first 60 min and lose thymidine nucleotides and dTDP-sugar pools at a faster rate for the first 15 min than those of the parent cells, but the dTDP-sugar pool in the parent cells is maintained at high concentration for the first 90 min during thymine starvation. In the recovery of cell growth after re-addition of thymine into the thymine-deprived culture, parent cells recommence growth immediately, but the mutant cells (Ter-15) show a lag-phase for 45 min after which time their growth recommences. The rate of dTTP synthesis for the first 10 to 15 min after re-addition of thymine to thymine-deprived cultures of parent and mutant (Ter-15) cells is three-fold higher than that of thymine nondeprived culture (control), but the rates of dTMP and dTDP-sugar syntheses are the same as those of the control. The total DNA synthesis after re-addition of thymine is equal to that of the control, and the period of thymine starvation other than the number of viable cells during thymine starvation plays an important role. After separation of the filament cells from normal-sized cells by sucrose gradient centrifugation, the initial rate of DNA synthesis of filament cells is three-fold faster than that of normal-sized cells. These results show that the dependency of DNA synthesis upon dTTP concentration is maintained after re-addition of thymine into thymine-deprived culture.