Excess dTMP is toxic and mutagenic with exponentially growing dTMP efficient uptaking yeast strains 831 rho+ and 833 rho. The respiratory deficient strain 833 exhibits a tenfold sensitivity to the genotoxicity of excess dTMP. Mutant yield in the forward mutation system CAN1----can1 after dTMP excess is comparable to that found after irradiation with UV254nm. Excess dTMP is a poor mutagen in stationary phase cells of both strains. Mutagenicity of excess dTMP is not found in an ochre mutant allele (ade2-1). Exposure of exponentially growing cells to other deoxyribonucleotides (dCMP, dAMP, and dGMP) reveal these nucleotides to have mutagenic potential as well.