Escherichia coli WP2 trpE65(ochre), when grown to stationary phase and plated on glucose salts medium, gave rise to slow growing tryptophan-independent mutants which formed increasing numbers of visible colonies from about day 6 onwards. These mutants were neither revertants at the ochre codon nor mutants at the tRNA suppressor loci normally found in this strain with logarithmic phase mutagenesis. The yield of mutants was not affected by the presence of the following alleles, umuC122, lexA102, polA1, recA1, recA56 or del(srlR-recA)306, except that in the three recA-defective strains, mutant colonies were initially slower to appear, possibly reflecting a lower viability in the inoculum. Stationary-phase spontaneous mutation in bacteria carrying on ochre mutation is thus a distinct and specific process that does not require the SOS system, or UvrA protein or DNA polymerase I. It may reflect the occurrence of a type of non-bulky DNA damage with altered base pairing specificity. In 3 out of 4 experiments with a strain carrying recA441 plus lexA51(Def) the rate of stationary-phase mutagenesis was elevated suggesting that there may be an additional component requiring an activated SOS system.