Activation of maturation-promoting factor at the onset of mitosis requires the tyrosine dephosphorylation of one of its components, the cdc2 protein kinase. cdc25 is the specific tyrosine phosphatase that activates cdc2. We find that Xenopus oocytes contain a relative of cdc25, p72. In Xenopus embryos the abundance of p72 does not oscillate during the cell cycle. However, p72 directly associates with cdc2-cyclin B in a cell cycle-dependent manner, reaching a peak at M phase. The M phase kinase that associates with p72 is catalytically active. These results suggest that the mechanism by which cdc25 triggers cdc2 activation involves a periodic physical association between cdc25 and the cyclin B-cdc2 complex and also that mitotic control can be affected by mechanisms other than transcriptional regulation of the cdc25 gene.