Cytokinesis in bacteria is initiated by polymerization of the tubulin homologue FtsZ into a circular structure at midcell, the Z-ring. This structure functions as a scaffold for all other cell division proteins. Several proteins support assembly of the Z-ring, and one such protein, SepF, is required for normal cell division in Gram-positive bacteria and cyanobacteria. Mutation of sepF results in deformed division septa. It is unclear how SepF contributes to the synthesis of normal septa. We have studied SepF by electron microscopy (EM) and found that the protein assembles into very large (∼50 nm diameter) rings. These rings were able to bundle FtsZ protofilaments into strikingly long and regular tubular structures reminiscent of eukaryotic microtubules. SepF mutants that disturb interaction with FtsZ or that impair ring formation are no longer able to align FtsZ filaments in vitro, and fail to support normal cell division in vivo. We propose that SepF rings are required for the regular arrangement of FtsZ filaments. Absence of this ordered state could explain the grossly distorted septal morphologies seen in sepF mutants.