The presence of an adaptive response to low doses of alkylating chemicals in the phototrophic bacteria Rhodobacter sphaeroides and R.capsulatushas been studied. Results obtained show that both strains display this repair response against the challenge doses of N-methyl-N′-nitro-N-nitroso-guanidine (MNNG), when they are pretreated with low doses of this compound for 120 min. The adaptive response of both R.sphaeroides and R.capsulatus induced an increase of cell survival and a decrease of mutagenesis in the MNNG-pre-treated cells. Furthermore, the MNNG-induced adaptive repair also gives protection to diethylsulphate and ethylme-thanesulphonate in both phototrophic bacteria. Finally, the MNNG-promoted adaptive response is sensitive to inhibitors of protein synthesis such as chloramphenicol, indicating that this DNA repair mechanism needs protein synthesis in R.sphaeroides and R.capsulatus, in a way similar to that which occurs in Escherichia coli. © 1988 IRL Press Ltd.