Mx proteins form a subfamily of the dynamin-like GTPases, which have well established roles in cellular trafficking. Some Mx proteins (e.g., human MxA) have antiviral activity and are tightly regulated by type I IFNs. Others (e.g., human MxB) lack antiviral activity and are thought to have normal cellular functions that remain undefined. Consistent with this hypothesis, we report that MxB is expressed without IFN treatment. MxB seems to be exclusively extranuclear and is concentrated at the cytoplasmic face of nuclear pores, suggesting a role in their regulation. We find that expression of dominant negative (GTPase-defective) MxB mutants efficiently blocks nuclear import and causes a delay in G(1)/S cell-cycle progression. MxB depletion using RNA interference (RNAi) leads to a similar cell-cycle defect but does not block nuclear import. MxB therefore seems not to be required for nuclear import per se but may instead regulate its efficiency and/or kinetics. These studies indicate an unexpected role for a dynamin-like protein in nucleocytoplasmic trafficking and suggest that a related function might be usurped by its antiviral relatives.