FtsZ, the major cytoskeletal component of the bacterial cell-division machine, assembles into a ring (the Z-ring) that contracts at septation. FtsZ is a bacterial homolog of tubulin, with similar tertiary structure, GTP hydrolysis, and in vitro assembly. We used green fluorescent protein-labeled FtsZ and fluorescence recovery after photobleaching to show that the E. coli Z-ring is extremely dynamic, continually remodeling itself with a half-time of 30 s. ZipA, a membrane protein involved in cell division that colocalizes with FtsZ, was equally dynamic. The Z-ring of the mutant ftsZ84, which has 1/10 the guanosine triphosphatase activity of wild-type FtsZ in vitro, showed a 9-fold slower turnover in vivo. This finding implies that assembly dynamics are determined primarily by GTP hydrolysis. Despite the greatly reduced assembly dynamics, the ftsZ84 cells divide with a normal cell-cycle time.