In order to investigate the mutational events induced by the very potent genotoxic agent 7-methoxy-2-nitronaphtho[2,1-b]furan (R7000), we evaluated its mutagenic potency in a battery of his- mutants of Salmonella typhimurium designed to study mutagenic specificity (Levin and Ames, 1986). Using this system we could show that R7000 is able to induce base-pair substitutions, mainly GC----TA transversions and, perhaps to a lower extent, AT----TA transversions. In addition, our results suggest that this chemical may be a very efficient inducer of base-pair deletions/insertions resulting in frameshifts. Whereas the induction of base-pair substitutions by R7000 is dependent on the presence of plasmid pKM101 carrying the mucA,B operon, the induction of base-pair deletions/insertions is independent of the presence of the plasmid.