The yeast Saccharomyces cerevisiae appears to possess a single mutagenic or "error prone" pathway for the repair of UV damage, probably involving the functions of at least seven genes; rev1, rev2, rev3 ( Lemontt 1971a), rad6, rad8, rad9 and rad18 ( Lawrence et al. 1974; present results). Strains carrying rad6 are the most sensitive to the lethal effects of UV light in this group and double mutants carrying rad6 and either rev1, rev3, rad9 or rad18 are no more sensitive than this single mutant strain. rev3 rad6 double mutant diploids failed to show any UV-induced reversion of the normally highly revertible ochre allele cyc1–9 , even though a total of more than 2.5 x 10⁹ viable cells was examined, suggesting that strains of this kind are entirely UV-immutable; spontaneous revertants could be recovered, however.—The rad6 and rev3 gene products would appear to be necessary for all kinds of mutagenic events at all sites within the genome, but the products of the other genes that act in the "errorprone" pathway hage a more restricted role and are involved in the production of only some kinds of mutations. It is suggested that such selectivity arises from the interaction of some repair enzymes with specific nucleotide sequences.