Synapsin I is a neuron-specific phosphoprotein localized to the cytoplasmic surface of synaptic vesicles. This phosphoprotein is a major substrate for cyclic AMP-dependent and calcium/calmodulin-dependent protein kinases. Its state of phosphorylation can be altered both in vivo and in vitro by a variety of physiological and pharmacological manipulations known to affect synaptic function. Recent direct evidence suggests that it may be involved in the regulation of neurotransmitter release from the nerve terminal. In the nerve terminal, synaptic vesicles are embedded in a cytoskeletal network, consisting in part of actin. We report here the ability of the dephospho-form of synapsin I to bundle F-actin. This bundling activity is reduced when synapsin I is phosphorylated by cAMP-dependent protein kinase and virtually abolished when it is phosphorylated by calcium/calmodulin-dependent protein kinase II or by both kinases. These results, demonstrating an interaction of synapsin I with actin in vitro, support the possibility that synapsin I is involved in clustering of synaptic vesicles at the presynaptic terminal and that the phosphorylation of synapsin I may be involved in regulating the translocation of synaptic vesicles to their sites of release.