DNA mismatch repair (MMR) is integral to the maintenance of genomic stability and more recently has been demonstrated to affect apoptosis and cell cycle arrest in response to a variety of adducts induced by exogenous agents. Comparing Msh2-null and wildtype mouse embryonic fibroblasts (MEFs), both primary and transformed, we show that Msh2 deficiency results in increased survival post-UVB, and that UVB-induced apoptosis is significantly reduced in Msh2-deficient cells. Furthermore, p53 phosphorylation at serine 15 is delayed or diminished in Msh2-deficient cells, suggesting that Msh2 may act upstream of p53 in a post-UVB apoptosis or growth arrest response pathway. Taken together, these data suggest that MMR heterodimers containing Msh2 may function as a sensor of UVB-induced DNA damage and influence the initiation of UVB-induced apoptosis, thus implicating MMR in protecting against UV-induced tumorigenesis.