FtsZ is the polymer-forming protein of bacterial cell division. It is part of a ring in the middle of the dividing cell that is required for constriction of cell membrane and cell envelope to yield two daughter cells. FtsZ is a GTPase and is the only bacterial protein showing significant sequence homology to the eukaryotic tubulins. FtsZ can polymerize into tubes, sheets, and rings in vitro and is ubiquitous in eubacteria and archaea. Full-length FtsZ1 from Methanococcus jannaschii has been over expressed in Escherichia coil, employing the hyperthermophilic properties of the protein. Crystals grown from PEG400 and ethanol belong to spacegroup I2(1)3 with a = b = c = 159.1 Angstrom. Isomorphous replacement using one Hg derivative yielded a interpretable electron density map at 4 Angstrom resolution. The structure for residues 23-356 and one GDP hats been refined to an R-free of 0.28 (R-f = 0.20) at 2.8 Angstrom resolution. FtsZ consists of two domains with a connecting core helix. The N-terminal domain and the core helix contain all residues involved in nucleotide binding and resemble the fold of dinucleotide-binding proteins. The structures of tubulin and FtsZ show striking similarity; together with the functional similarities, this provides a strong indication that FtsZ is a true homolog of tubulin.