Dynamin, a crucial factor in endocytosis, is a member of a family of GTPases that participates in membrane fission. It was initially proposed to act as a machine that constricts and cuts the neck of nascent vesicles in a GTP-hydrolysis-dependent reaction, but subsequent studies suggested alternative models. Here we monitored the effect of nucleotides on dynamin-coated lipid tubules in real time. Addition of GTP, but not of GDP or GTP-gammaS, resulted in twisting of the tubules and supercoiling, suggesting a rotatory movement of the helix turns relative to each other during GTP hydrolysis. Rotation was confirmed by the movement of beads attached to the tubules. Twisting activity produced a longitudinal tension that was released by tubule breakage when both ends of the tubule were anchored. Fission also occurred when dynamin and GTP were added to lipid tubules that had been generated from liposomes by the motor activity of kinesin on microtubules. No fission events were observed in the absence of longitudinal tension. These findings demonstrate a mechanoenzyme activity of dynamin in endocytosis, but also imply that constriction is not sufficient for fission. At the short necks of endocytic vesicles, other factors leading to tension may cooperate with the constricting activity of dynamin to induce fission.