Phosphorylation is one of the major mechanisms by which the activity of transcription factors can be regulated. We have investigated the role of phosphorylation in the regulation of the transcription factor CREM. We show that the CREMtau activator is phosphorylated on multiple serine and threonine residues in vivo. Stimulation of various signal transduction pathways by forskolin, TPA or Ca2+ ionophore leads to enhanced phosphorylation of serine 117, concomitant with an increase in the transactivation potential of CREMtau. We have identified multiple kinases that can also phosphorylate S117 in vitro. Moreover, we show that casein kinase I and II cooperatively phosphorylate CREMtau on multiple residues, eliciting enhanced DNA binding. Cooperative phosphorylation is also observed with other kinases. These results show that the activity of CREMtau is regulated by multiple phosphorylation events, suggesting that CREM could be considered as a nuclear effector where signalling pathways may converge and/or cross-talk.