Mutation frequency decline (MFD) is an irreversible loss of newly-induced suppressor mutations occurring in excision-proficient Escherichia coli during a short period of incubation in minimal medium before plating on broth- or Casamino acids-enriched selective agar. It is known that MFD of UV-induced mutations may occur before DNA containing pre-mutagenic lesions is replicated, but we conclude that MFD can also occur after the damaged DNA has been replicated on the basis of the following evidence. (1) Mutation fixation in rich medium (i.e., loss of susceptibility to mutation frequency decline) with ethyl methanesulphonate mutagenesis begins immediately, whereas with UV it is delayed for 20--30 min. (2) The delay in mutation fixation after UV can be explained neither by inhibition of DNA replication nor by a delay in the appearance of error-prone repair activity in the irradiated population. (3) MFD at later times after UV irradiation is more rapid and is less strongly inhibited by caffeine than is MFD immediately after irradiation. (4) Excision is virtually complete 20 min after 3 J m-2 UV but at that time virtually all mutations are still susceptible to MFD. We have presented evidence elsewhere that in bacteria there is an alternative error-free excision-dependent type of post-replication repair of potentially mutagenic daughter strand gaps. We suggest that this process is inhibited at tRNA loci in the presence of nutrient broth or Casamino acids, possibly because of a broth-dependent change in the structure of the single-stranded region including the tRNA locus.