FtsZ is a prokaryotic homologue of the eukaryotic cytoskeletal protein tubulin and plays a central role in prokaryotic cell division. Both FtsZ and tubulin are known to pass through cycles of polymerization and depolymerization, but the structural mechanisms underlying this cycle remain to be determined. Comparison of tubulin structures obtained in different states has led to a model in which the tubulin monomer undergoes a conformational switch between a "straight" form found in the walls of microtubules and a "curved" form associated with depolymerization, and it was proposed recently that this model may apply also to FtsZ. Here, we present new structures of FtsZ from47 Aquifex aeolicus,47 Bacillus subtilis, Methanococcus jannaschii and Pseudomonas aeruginosa that provide strong constraints on any proposed role for a conformational switch in the FtsZ monomer. By comparing the full range of FtsZ structures determined in different crystal forms and nucleotide states, and in the presence or in the absence of regulatory proteins, we find no evidence of a conformational change involving domain movement. Our new structural data make it clear that the previously proposed straight and curved conformations of FtsZ were related to inter-species differences in domain orientation rather than two interconvertible conformations. We propose a new model in which lateral interactions help determine the curvature of protofilaments.