FtsZ forms a cytokinetic ring, designated the Z ring, that directs cytokinesis in prokaryotes. It has limited sequence similarity to eukaryotic tubulins and, like tubulin, it has GTPase activity and the ability to assemble into various structures including protofilaments, bundles and minirings. By using both electron microscopy and sedimentation, we demonstrate that FtsZ from Escherichia coli undergoes a strictly GTP-dependent polymerization and the polymers disappear as the GTP is consumed. Thus, FtsZ polymerization, like that of tubulin, is dynamic and regulated by GTP hydrolysis. These results provide the basis for the dynamics of the Z ring and favor a model in which the Z ring is formed by a nucleation event.