CD4+CD8+ double-positive (DP) thymocytes, which are extremely sensitive to apoptosis, specifically up-regulate Bcl-xL to extend their lifespan. Deletion of the Bcl-xL gene leads to premature apoptosis of the thymocytes. In this study, we show that stabilization of beta-catenin, a critical coactivator for T cell factor (TCF), enhances DP thymocyte survival via up-regulating Bcl-xL. Spontaneous or glucocorticoid-induced thymocyte apoptosis was associated with reduced levels of beta-catenin and Bcl-xL. Transgenic expression of a stabilized beta-catenin protected DP thymocytes from both spontaneous and glucocorticoid-induced apoptosis, resulting in significantly increased thymic cellularity. Compared with the wild-type mice, both protein and transcript levels of Bcl-xL were significantly increased in thymocytes of beta-catenin transgenic mice. In addition, TCF-1 as well as beta-catenin were able to stimulate transcriptional activity of the reporter driven by a Bcl-xL promoter. beta-Catenin/TCF is thus able to act as a signal to up-regulate Bcl-xL levels in DP thymocytes, resulting in their enhanced survival.