Dynamin, a 100 kDa GTPase, is essential for receptor mediated endocytosis and synaptic vesicle recycling; however its mechanism of action is unknown. The requirement for dynamin was first elucidated by the discovery that the shibire gene product in Drosophila melanogaster was homologous to mammalian dynamin-1 (1,2). The shibire flies exhibit a depletion of synaptic vesicles and an accumulation of collared clathrin-coated pits at the plasma membrane of their nerve termini (3). It was later demonstrated that endocytosis was inhibited by the overexpression of dominant negative mutants of dynamin (4,5), and that purified dynamin can self-associate to form spirals which resemble the collars of shibire and structures seen in synaptosomes treated with GTPγS (6,7). These observations led to the speculation that dynamin pinches the clathrin-coated bud from the plasma membrane. In support of this hypothesis, we show that purified recombinant dynamin can bind to a lipid bilayer in a regular and repeating pattern to form helical tubes which vesiculate upon the addition of GTP.