We have studied the assembly and GTPase of purified FtsZ from the hyperthermophilic archaeon Methanococcus jannaschii, a structural homolog of eukaryotic tubulin, employing wild-type FtsZ, FtsZ-His6 (histidine-tagged FtsZ), and the new mutants FtsZ-W319Y and FtsZ-W319Y-His6, with light scattering, nucleotide analyses, electron microscopy, and image processing methods. This has revealed novel properties of FtsZ. The GTPase of archaeal FtsZ polymers is suppressed in Na+-containing buffer, generating stabilized structures that require GDP addition for disassembly. FtsZ assembly is polymorphic. Archaeal FtsZ(wt) assembles into associated and isolated filaments made of two parallel protofilaments with a 43 A longitudinal spacing between monomers, and this structure is also observed in bacterial FtsZ from Escherichia coli. The His6 extension facilitates the artificial formation of helical tubes and sheets. FtsZ-W319Y-His6 is an inactivated GTPase whose assembly remains regulated by GTP and Mg2+. It forms two-dimensional crystals made of symmetrical pairs of tubulin-like protofilaments, which associate in an antiparallel array (similarly to the known Ca2+-induced sheets of FtsZ-His6). In contrast to the lateral interactions of microtubule protofilaments, we propose that the primary assembly product of FtsZ is the double-stranded filament, one or several of which might form the dynamic Z ring during prokaryotic cell division.