The cdc2+ protein kinase (pp34) is found to be phosphorylated on tyrosine as well as serine and threonine residues in exponentially growing Schizosaccharomyces pombe. At mitosis, the level of pp34 phosphorylation on both threonine and tyrosine residues decreases. The single detectable site of tyrosine phosphorylation in pp34 has been mapped to Tyr 15, a residue within the presumptive ATP-binding domain. Substitution of this tyrosine by phenylalanine advances cells prematurely into mitosis, establishing that tyrosine phosphorylation/dephosphorylation directly regulates pp34 function.