The fluorinated pyrimidines 5-fluorouracil (5FU) and 5-fluorocytosine (5FC) induce the cytoplasmic petite mutation in the yeast Saccharomyces cerevisiae with high efficiency. It was found that in order to induce the mutation, 5FC must first be deaminated to 5FU. However, mutagenesis does not depend on the further conversion of 5FU to its deoxyriboside (5FUDR) and subsequent blockade of intracellular thymidine synthesis, since 5FUDR itself was found not to be mutagenic, and 5FU-induced mutagenesis was not antagonised by supplying thymidine monophosphate (dTMP) to a dTMP permeable strain. In any case, observations of the molecular changes accompanying petite induction in log phase cells ruled out the possibility that mutagenesis resulted simply from the dilution out of replication blocked mitDNA molecules, since the appearance of mutants coincided with the synthesis of altered mitDNA molecules. In different strains, the resulting defective molecules were either maintained, giving rise to suppressive rho- petites, or completely degraded, to give pure clones of neutral rho0 mutants. It is suggested that this degradative process was a conseuqence of the incorporation of 5FU into RNA.