Purpose: Previous studies indicate that the Wnt/beta-catenin signalling pathway is active and functional during murine lens development. In this study we investigated the consequences of constitutively activating the pathway in lens during development. Methods: We conditionally mutated beta-catenin (Catnb) and adenomatous polyposis coli (Apc) genes to activate Wnt/beta-catenin signalling, using two Cre lines that are active in whole lens (MLR10) or only in differentiated fibers (MLR39), from E13.5. Lens phenotype in mutant lenses was investigated by histology, immunohistochemistry, BrdU labelling, quantitative RT-PCR arrays and by TUNEL. Results: Only intercrosses with MLR10 resulted in ocular phenotypes, indicating Wnt/beta-catenin signalling functions in lens epithelium and during early fibre differentiation. Mutant lenses were characterised by increased progression of epithelial cells through the cell cycle, as shown by BrdU labelling, and phospho-histone 3 and cyclin D1 labelling, and maintenance of epithelial phenotype (E-cadherin and Pax6 expression) in the fiber compartment. Fiber cell differentiation was delayed as shown by reduced expression of c-maf and beta-crystallin and delay in expression of the CDKI, p57(kip2). From E13.5 there were numerous cells undergoing apoptosis and by E15.5 there was evidence for epithelial-mesenchymal transition with numerous cells expressing alpha-smooth muscle actin. Quantitative PCR analyses revealed large changes in expression of Wnt target genes (Lef1, Tcf7, Brachyury, cyclin D1), Wnt inhibitors (Wif1, Dkk1, Nkd1, Frzb) and also several Wnts (Wnt6, Wnt10a, Wnt8b, Wnt11). Conclusions: These data indicate that the Wnt/beta-catenin pathway, plays key roles in regulating proliferation of lens stem/progenitor cells and during early stages of fiber cell differentiation.