We have recently reported the isolation of a class of mutants (called thy-) that is both resistant to arabinosyl cytosine and auxotrophic for thymidine. thy- mutants have a 5- to 10-fold elevated pool of dCTP and are deficient in the synthesis of dTTP as an apparent consequence of a single mutation in the gene for ribonucleoside-diphosphate reductase (2'-deoxyribonucleoside-diphosphate:oxidized-thioredoxin 2'-oxidoreductase, EC 1.17.4.1). Here we show that three independent thy- lines have a 5- to 50-fold higher frequency and rate of spontaneous mutation for two genetic markers, 6-thioguanine resistance and ouabain resistance. The higher rate of mutation is site specific because two other genetic markers, reversion of proline auxotrophy to proline prototrophy and emetine resistance, are unaffected. Ouabain- and 6-thioguanine-resistant mutations occur at a much lower rate in revertants of thy- to the wild-type state, so the increased rate of mutation is the consequence of the thy- mutation. Both the increased mutational rate and the increased intracellular pools of dCTP are dominant or codominant in hybrid cells, and alterations of the ratio of the pools of dCTP to dTTP in thy- 49 produce corresponding changes in the rate of mutation. Thus, thy- is a mutator gene in Chinese hamster ovary cells, apparently as a consequence of the imbalance of deoxynucleoside triphosphate pools created by the expanded pool of dCTP.