AbstractBacterial cell division depends on the formation of a cytokinetic ring structure, the Z-ring. The bacterial tubulin homologue FtsZ is required for Z-ring formation. FtsZ assembles into various polymeric formsin vitro, indicating a structural role in the septum of bacteria. We have used recombinant FtsZ1 protein fromM. jannaschiito produce helical tubes and sheets with high yield using the GTP analogue GMPCPP [guanylyl-(α,β)-methylene-diphosphate]. The sheets appear identical to the previously reported Ca++-induced sheets of FtsZ fromM. jannaschiithat were shown to consist of ‘thick’-filaments in which two protofilaments run in parallel. Tubes assembled either in Ca++or in GMPCPP contain filaments whose dimensions indicate that they could be equivalent to the ‘thick’-filaments in sheets. Some tubes are hollow but others are filled by additional protein density. Helical FtsZ tubes differ from eukaryotic microtubules in that the filaments curve around the filament axis with a pitch of ~ 430 Å for Ca++-induced tubes or 590–620 Å for GMPCPP. However, their assemblyin vitroas well-ordered polymers over distances comparable to the inner circumference of a bacterium may indicate a rolein vivo. Their size and stability make them suitable for use in motility assays.