The small Suc1/Cks protein is a ubiquitous subunit of Cdk/cyclin complexes, but its precise function has remained unclear. We have isolated a Xenopus homolog, Xe-p9, of the Suc1/Cks protein by virtue of its ability to rescue a fission yeast mutant that enters mitosis prematurely. To assess its functional role in cell cycle control, we have both overexpressed p9 in Xenopus egg extracts and immunodepleted the protein from these extracts. We found that addition of recombinant His6-p9 to egg extracts results in a pronounced delay of mitosis that can be attributed to an inhibition of the tyrosine dephosphorylation of the inactive Cdc2/cyclin B complex. In immunodepletion studies, we observed that the consequences of removing p9 from egg extracts depend on the stage of the cell cycle. Specifically, in the case of interphase extracts, the removal of p9 abolishes the entry into mitosis as a result of a failure in the activation of the Cdc2/cyclin B complex by tyrosine dephosphorylation. Furthermore, mitotic extracts lacking p9 fail to exit mitosis because of a defect in the destruction of cyclin B. Collectively, these results indicate that p9 has multiple essential roles in the cell cycle by governing the interaction of the Cdc2/cyclin B complex with both positive and negative regulators.