SummaryWe performed comparative and mutational analyses to define more comprehensively the repertoire of genes involved in cyanobacterial cell division. Genes ftsE , ftsI , ftsQ , ftsW , and (previously recognized) ftsZ , minC , minD , minE and sulA were identified as homologues of cell division genes of Gram-negative and Gram-positive bacteria. Transposon mutagenesis of Synechococcus elongatus PCC 7942 identified five additional genes, cdv1 , cdv2 , cdv3 , ftn6 and cikA , involved in cell division. cdv1 encodes a presumptive periplasmic peptidyl-prolyl cis -trans isomerase. cdv2 has similarity to ylmF which, like divIVA , lies within the Gram-positive bacterial ylm gene cluster whose members have functions associated with division. Conservation of other ylm genes in cyanobacteria suggests that cyanobacteria and Gram-positive bacteria share specific division proteins. Two ylm homologues are also found in algal and plant genomes. cdv3 has low but significant similarity to divIVA , suggesting that minE and cdv3 both mediate division-site determination in cyanobacteria. In contrast, Grampositive bacteria lack minE , and (Gram-negative) proteobacteria lack divIVA . ftn6 , of unknown function, and the circadian input kinase, cikA , are specific to cyanobacteria. In S. elongatus , unlike in other bacteria, FtsZ rings are formed at sites occupied by nucleoids. Thus, the division machinery of cyanobacteria differs in its composition and regulation from that of Gram-negative and Gram-positive bacteria.