The dynamin proteins have been associated with the process of endocytosis for many years. Until recently it was considered that yeast dynamin-related proteins did not play a role in endocytosis and the proposed scission function of dynamin was attributed to another group of proteins, the amphiphysins. However, it has now been shown that the yeast dynamin-like protein Vps1 shows a transient burst of localization to sites of endocytosis. Vps1 assembles at cortical sites at the time when actin polymerization is proposed to drive plasma membrane invagination. In concert with the amphiphysins Vps1 is then thought to function in the scission step to release a formed vesicle. It was shown that a mutation preventing self assembly of Vps1 caused a defect in endocytosis but not in other functions with which Vps1 is associated. Using electron microscopy we now show that this mutation I649K, corresponding to I690K in human Dyn1, causes formation of long endocytic invaginations. The data suggest that an ability of Vps1 to self assemble and to thereby stimulate its GTPase activity is critical for the 'pinching-off' stage of endocytosis to form a vesicle.