||Dendritic spines are small protrusions from dendrites, whose morphological changes are thought to play important roles in synaptic plasticity. Recent studies have revealed multiple signaling pathways that regulate dendritic spine development, among which are members of the Eph receptor tyrosine kinase family and their ephrin ligands. In particular, while activation of EphB by ephrinBs stimulates the formation of mature dendritic spines, activation of EphA4 by ephrinAs induces dendritic spine retraction. The mechanism by which Eph-ephrin regulates the dendritic spine dynamics remain elusive and have drawn much attention. Previous studies in our laboratory have revealed an important function of cyclin-dependent kinase 5 (Cdk5) in EphA4-dependent spine retraction, raising the intriguing possibility that Cdk5 is also involved in EphB-dependent dendritic spine morphogenesis. It is thus of interest to examine whether Cdk5/p35 take part in the EphB-dependent dendritic spine morphogenesis. As a first step to explore the functional roles of Cdk5/p35 in dendritic spine development, we examined the interaction between EphB2 and Cdk5/p35. We found that EphB2 interacts with Cdk5/p35 complex and phosphorylates Cdk5. Interestingly, we demonstrated that treatment of cultured hippocampal neurons with ephrinB1 increases the number of mature spines whereas such effect is abolished in p35 knock-down neurons. In addition, we found that EphB2-dependent recruitment of cell surface receptor protein subunit, GluR2, is also interrupted in p35 knock-out neurons. Taken together, our findings suggested that EphB2-dependent dendritic spine morphogenesis is regulated by Cdk5/p35.