p13(suc1) from fission yeast is a member of the CDC28 kinase specific (CKS) class of cell-cycle control proteins, that includes CKS1 from budding yeast and the human homologues CksHs1 and CksHs2. p13(suc1) participates in the regulation of p34(cdc2), a cyclin-dependent kinase controlling the G1-S and the G2-M transitions of the cell cycle. The CKS proteins are believed to exert their regulatory activity by binding to the kinase, in which case their function may be governed by their conformation or oligomerization state. Previously determined X-ray structures of p13(suc1), CksHs1 and CksHs2 show that these proteins share a common fold but adopt different oligomeric states. Monomeric forms of p13(suc1) and CksHs1 have been solved. In addition, CksHs2 and p13(suc1) have been observed by X-ray crystallography in assemblies of strand-exchanged dimers. Analysis of various assemblies of the CKS proteins, as found in different crystal forms, should help to clarify their role in cell-cycle control. We report the X-ray crystal structure of p13(suc1) to 1.95 A resolution in space group C2221. It is present in the crystals as a strand-exchanged dimer. The overall monomeric fold is preserved in each lobe of the dimer but a single beta-strand (Ile94-Asp102) is exchanged between the central beta-sheets of each molecule. Strand exchange, which has been observed for p13(suc1) in two different space groups, and for CksHs2, is now confirmed to be an intrinsic feature of the CKS family. A switch between levels of assembly may serve to coordinate the function of the CKS proteins in cell-cycle control.