B cell-specific gene ablation of Notch2 results in the loss of the Marginal Zone (MZ) B cell lineage. To analyze the effects of constitutive Notch2 signaling in B cells, we have generated a transgenic mouse strain allowing the conditional expression of a constitutively active, intracellular form of Notch2 (Notch2IC). Expression of Notch2IC at the earliest developmental stages of the B cell lineage completely abolished B cell generation and led to the development of ectopic T cells in the bone marrow, showing that Notch2IC is acting redundantly with Notch1IC in driving ectopic T cell differentiation. In B cells clearly committed to the B cell lineage induction of Notch2IC drove all cells towards the MZ B cell compartment at the expense of Follicular B cells. Notch2IC-expressing B cells reflected the phenotype of wild type MZ B cells with respect to their localization in the MZ, the expression of characteristic surface markers, their enhanced proliferation after stimulation and increased basal activity of Akt, Erk and Jnk. Notch2IC-driven MZ B cell generation in the spleen was achieved even in the absence of CD19. Our results implicate that a constitutive Notch2 signal in T1 B cells is sufficient to drive MZ B cell differentiation.