Using mutants (tgt, mnmA(asuE, trmU), mnmE(trmE), miaA, miaB, miaE, truA(hisT), truB) of either Escherichia coli or Salmonella enterica serovar Typhimurium and the trm5 mutant of Saccharomyces cerevisiae, we have analyzed the influence by the modified nucleosides Q34, mnm(5)s(2)U34, ms(2)io(6)A37, Psi39, Psi55, m(1)G37, and yW37 on -1 frameshifts errors at various heptameric sequences, at which at least one codon is decoded by tRNAs having these modified nucleosides. The frequency of -1 frameshifting was the same in congenic strains only differing in the allelic state of the various tRNA modification genes. In fact, in one case (deficiency of mnm(5)s(2)U34), we observed a reduced ability of the undermodified tRNA to make a -1 frameshift error. These results are in sharp contrast to earlier observations that tRNA modification prevents +1 frameshifting suggesting that the mechanisms by which -1 and +1 frameshift errors occur are different. Possible mechanisms explaining these results are discussed.