Spindle morphogenesis and dynamics follow an orderly sequence of events coupled to the oscillatory activation of cyclin-dependent kinase (CDK). Using S. cerevisiae, we have addressed the requirement of CDK for phosphorylation of the spindle midzone component Ase1p and its significance to spindle assembly. Ase1p is related to human PRC1, a protein negatively regulated by CDK until late mitosis, when it is required for central spindle organization and cytokinesis. By contrast, we show here that Ase1p phosphorylation by CDK promotes correct spindle assembly. Indeed, Ase1p phosphorylation coincident with spindle assembly requires Clb5p, Clb3p and Clb4p. Moreover, in clb5Δ cells, Ase1p recruitment and the kinetics of spindle formation were perturbed. These phenotypes were enhanced in a cdc28-4 clb5Δ mutant to the extent that midzone disruption resulted in transient breaks of the short spindle. By contrast, clb3Δ clb4Δ cells delayed spindle assembly downstream to Ase1p recruitment. Expression of Ase17Dp that mimics the phosphorylated state restored timely recruitment in clb5Δ cells and fully rescued the corresponding spindle phenotypes. Finally, Ase17Dp partially suppressed the spindle assembly delay in clb3Δ clb4Δ cells. Thus, Ase1p phosphorylation by CDK promotes the assembly and stability of the mitotic spindle. It follows that CDK may differentially alter the functionality of members of the Ase1p/PRC1 family to place their distinct roles in their respective stage-specific contexts, a further factor of complexity in the organization of pathways promoting spindle assembly and dynamics.