To determine the function of immunoglobulin (Ig)α immunoreceptor tyrosine–based activation motif (ITAM) phosphorylation, we generated mice in which Igα ITAM tyrosines were replaced by phenylalanines (Igα^FF/FF). Igα^FF/FFmice had a specific reduction of B1 and marginal zone B cells, whereas B2 cell development appeared to be normal, except that λ1 light chain usage was increased. The mutants responded less efficiently to T cell–dependent antigens, whereas T cell–independent responses were unaffected. Upon B cell receptor ligation, the cells exhibited heightened calcium flux, weaker Lyn and Syk tyrosine phosphorylation, and phosphorylation of Igα non-ITAM tyrosines. Strikingly, when the Igα ITAM mutation was combined with a truncation of Igβ, B cell development was completely blocked at the pro-B cell stage, indicating a crucial role of ITAM phosphorylation in B cell development.