The GTPase dynamin is essential for numerous vesiculation events including clathrin-mediated endocytosis. Upon GTP hydrolysis, dynamin constricts a lipid bilayer. Previously, a three-dimensional structure of mutant dynamin in the constricted state was determined by helical reconstruction methods. We solved the nonconstricted state by a single-particle approach and show that the stalk region of dynamin undergoes a large conformational change that drives tube constriction.