The mutagenic consequences of damages to M13 mp19 RF DNA produced by singlet oxygen have been determined in a forward mutational system capable of detecting all classes of mutagenic events. When the damaged M13 mp19 RF DNA is used to transfect competent E. coli JM105 cells, a 16.6-fold increase in mutation frequency is observed at 5% survivors when measured as a loss of alpha-complementation. The enhanced mutagenicity is largely due to single-nucleotide substitutions, frameshift events and double-mutations. The single-nucleotide substitutions occur in the regulatory and in the structural part of the lacZ gene under the predominant form of a G:C to T:A transversion. The spectrum of mutations detected among the M13 lacZ phages surviving the singlet oxygen treatment is totally different from those appearing spontaneously. SOS induction mediated through u.v.-irradiation of bacteria leads to an increase of the mutation frequency in the M13 surviving to the singlet oxygen treatment. The mutation spectrum in this case is a mixture between those observed with the spontaneous mutants and the mutants induced by singlet oxygen. Lesions introduced in the M13 mp19 RF DNA can be partly repaired by the enzymatic machinery of the bacteria. It turns out that excision-repair and SOS repair are probably involved in the removal of these lesions by singlet oxygen.