The Ubc13 protein was recently identified for its unique role in ubiquitin (Ub) chain assembly at the Ub Lys-63 residue instead of the conventional Lys-48 residue. This activity requires Ubc13 to form a complex with Mms2 and indeed ubc13 and mms2 mutations have been shown elsewhere to be epistatic with respect to UV sensitivity. The MMS2 gene is known to be a member of the error-free DNA postreplication repair (PRR) pathway. By contrast, the Ub Lys-63 residue has been previously implicated in the error-prone PRR pathway, since yeast cells carrying the ubiK63R mutation are defective in UV-induced mutagenesis. In the present study, we attempted to define the role of UBC13 within the PRR pathway. We found that the ubc13 mutation is epistatic to mms2 and rad6, confirming that UBC13 belongs to the PRR-pathway. We also found that ubc13 is synergistic to the error-prone PRR pathway mutation rev3, indicating that UBC13 is in a pathway alternative to REV3 mutagenesis. The ubc13 mutant displays up to a 30-fold increase in the spontaneous mutation rate, and this increase is largely REV3 dependent. In addition, UV-induced mutagenesis is fully functional in the ubc13 mutant. These results together demonstrate that UBC13 is a member of the error-free PRR pathway. The involvement of UBC13 in cellular tolerance to DNA-damage is further implicated by our finding that the UBC13 transcript level is increased up to 6-fold in response to DNA-damage.