Glycogen synthase kinase‐3 (GSK‐3) is a protein serine kinase implicated in the cellular response to insulin. The enzyme is the mammalian homologue of the zeste‐white3 (shaggy) homeotic gene of Drosophila melanogaster and has been implicated in the regulation of the c‐Jun/AP‐1 transcription factor. In mammals this protein serine kinase is encoded by two related genes termed GSK‐3 alpha and beta. Here, we demonstrate that these two proteins and the fruit fly protein are phosphorylated on tyrosine in vivo. Moreover, GSK‐3 beta activity and function are shown to be dependent on tyrosine phosphorylation. The modified tyrosine residue is conserved in all members of the GSK‐3 family and is equivalent to that required for activity by mitogen‐activated protein (MAP) kinases. However, unlike MAP kinases, GSK‐3 is highly phosphorylated on tyrosine and thus active in resting cells.