The RhoA GTPase controls many cellular functions, including gene transcription and actin polymerization. Several lines of evidence suggest that Rho GTPases are required for B cell receptor (BCR) signaling, but whether RhoA is necessary has not been investigated. Here, we show that RhoA is activated, downstream of PI3K, in response to BCR stimulation and is important for BCR-dependent calcium flux and cell proliferation. A RhoA dominant-negative mutant strongly inhibited BCR-dependent calcium mobilization. The RhoA-specific inhibitor, C3 toxin, inhibited both BCR-dependent calcium flux and cell proliferation. RhoA is important for BCR-dependent synthesis of IP(3) by PLCgamma2, but is not required for tyrosine phosphorylation of PLCgamma2. BCR-dependent synthesis of phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P(2)) is inhibited in the absence of RhoA function. Providing exogenous PtdIns-4,5-P(2) restores BCR-dependent calcium flux in cells lacking functional RhoA. Our findings support a function for RhoA in BCR-dependent PtdIns-4,5-P(2) synthesis, PLCgamma2 activation, calcium mobilization, and cell proliferation.