Escherichia coli WP2 bacteria with an ochre amino acid auxotrophy show no evidence of growth during the first few days after plating at densities above 10⁸ on plates lacking the required amino acid. They lose viability for some days, and then a subpopulation recovers and there is cell turnover. At very low plating densities (around 10² per plate), almost every cell will eventually form a small but visible colony. At intermediate plating densities (10⁶ to 10⁷ per plate), there is an immediate increase in the number of viable bacteria. The results are consistent with a model that assumes that growth is dependent on trace amounts of tryptophan or a tryptophan-complementing substance and that death is due to extracellular toxic species in the medium, including active oxygen species. Mutations in mutT bacteria under these conditions result from incorporation of 7,8-dihydro-8-oxo-dGTP into DNA and thus largely reflect DNA synthesis associated with the increase in the number of viable cells at the initial density used (10⁷ per plate). We show that the increase in cell number and much of this DNA synthesis can be eliminated by the presence of 10⁸ scavenger bacteria and by removal of early-arising mutant colonies that release the required amino acid. The synthesis that remains is equivalent to less than a quarter of a genome per day and is marginally reduced, if at all, in a polA derivative. We cannot exclude the possibility that this residual DNA synthesis is peculiar to mutT bacteria due to transcriptional leakiness, although there is no evidence that this is a major problem in this strain. If such DNA synthesis also occurs in wild-type bacteria, it may well be important for adaptive mutation since use of a more refined agar in selective plates both eliminated the initial increase in cell number seen at low density (10⁷ per plate) and reduced the rate of appearance of mutants at plating densities above 10⁸ per plate.