The actin cytoskeleton plays a central role in many cell biological processes. The structure and dynamics of the actin cytoskeleton are regulated by numerous actin-binding proteins that usually contain one of the few known actin-binding motifs. WH2 domain (W ASP h omology domain-2 ) is a ∼35 residue actin monomer-binding motif, that is found in many different regulators of the actin cytoskeleton, including the β-thymosins, ciboulot, WASP (W iskott A ldrich s yndrome p rotein), verprolin/WIP (W ASP-i nteracting p rotein), Srv2/CAP (adenylyl c yclase-a ssociated p rotein) and several uncharacterized proteins. The most highly conserved residues in the WH2 domain are important in β-thymosin’s interactions with actin monomers, suggesting that all WH2 domains may interact with actin monomers through similar interfaces. Our sequence database searches did not reveal any WH2 domain-containing proteins in plants. However, we found three classes of these proteins: WASP, Srv2/CAP and verprolin/WIP in yeast and animals. This suggests that the WH2 domain is an ancient actin monomer-binding motif that existed before the divergence of fungal and animal lineages.