||Cyclin-dependent kinase 5 (Cdk5) is a unique member of the Cdk family that has been shown to play a diverse role in neural development. In the current study, we have demonstrated that p35, the activator of cyclin-dependent kinase 5 (Cdk5), associates with endophilin B1 (EndoB1) both in vitro and in vivo. Furthermore, we found that active Cdk5 phosphorylates EndoB1 in vitro, as well as in the EndoB1-overexpressing mammalian cells at Thr-145 of EndoB1. To examine the functional significance of this phosphorylation, we sought to examine how Cdk5 activity modulates the putative function of EndoB1. EndoB1 has been reported to bind to lipids, tubulate liposomes, in addition to exhibiting lysophosphatidic acid acyltransferase activity. Unlike endophilin A1, which interacts with endocytic proteins such as amphiphysin and dynamin to regulate endocytosis of synaptic vesicles, recent studies suggest that EndoB1 may be involved in intracellular protein trafficking and maintenance of mitochondria morphology. We observed that EndoB1 co-localizes with EEA1-positive vesicles in mammalian cells, suggesting that EndoB1 may function at early endosome and is involved in the regulation of the endolysosomal pathway. Using PC12 cells as a model, we investigated if EndoB1 may affect neurite outgrowth through modulating trafficking of internalized NGF receptors. We found that knock-down of EndoB1 in PC12 cells markedly inhibited the neurite outgrowth in NGF-treated PC12 cells, indicating that EndoB1 is required for NGF-induced neurite outgrowth. In addition, attenuation of neurite outgrowth was also observed in PC12 cells overexpressing EndoB1 phosphorylation mutant, suggesting that phosphorylation of EndoB1 by Cdk5 was required for neurite outgrowth in PC12 cells. Our findings indicate that phosphorylation of EndoB1 at Thr-T145 is essential for the function of EndoB1, and provide insights into understanding the role of Cdk5/p35 in intracellular trafficking pathways in neuronal development.