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Actin is one of the most abundant and conserved eukaryotic proteins. Remarkably, two prokaryotic homologs of actin, MreB and ParM, have only recently been identified. MreB and ParM polymerize into filaments and play important roles in the control of bacterial cell shape and plasmid segregation, respectively. Whereas the eukaryotic actins display a remarkable degree of conservation (e.g. no amino acid changes in muscle actin from chickens to humans), the two bacterial proteins have as much sequence similarity to each other (∼11% sequence identity) as they do to actin. It is possible that the interesting properties of eukaryotic F-actin may account for the unusual degree of conservation among the actins, whereas the bacterial proteins have had fewer constraints over the course of evolution. [Copyright &y& Elsevier]