Micrococcus radiodurans was highly resistant to the lethal effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) but it was sensitive to the mutagenic action of this chemical. The induction of mutation was not significantly modified by the culture growth phase. This last finding leads to the assumption that the mutation takes place at some distance from the replication fork. Moreover, a low concentration of MNNG induced mutations that were added to those subsequently obtained from a second exposure to a higher concentration of the alkylating agent. Thus, M. radiodurans does not seem to have an inducible error-free repair system for alkylation damage. Furthermore, incubation in the presence of chloramphenicol did not modify the mutation rate, indicating that protein synthesis is not involved in the mutagenic process.