Deamination of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) is a key plant-beneficial trait found in plant growth-promoting rhizobacteria (PGPR) and phytosymbiotic bacteria, but the diversity of the corresponding gene (acdS) is poorly documented. Here, acdS sequences were obtained by screening putative ACC deaminase sequences listed in databases, based on phylogenetic properties and key residues. In addition, acdS was sought in 71 proteobacterial strains by PCR amplification and/or hybridization using colony dot blots. The presence of acdS was confirmed in established AcdS1 bacteria and evidenced noticeably in Azospirillum (previously reported as AcdS), in 10 species of Burkholderia and six Burkholderia cepacia genomovars (which included PGPR, phytopathogens and opportunistic human pathogens), and in five Agrobacterium genomovars. The occurrence of acdS in true and opportunistic pathogens raises new questions concerning their ecology in plant-associated habitats. Many (but not all) acdS1 bacteria displayed ACC deaminase activity in vitro, including two Burkholderia clinical isolates. Phylogenetic analysis of partial acdS and deduced AcdS sequences evidenced three main phylogenetic clusters, each gathering pathogens and plant-beneficial strains of contrasting geographic and habitat origins. The acdS phylogenetic tree was only partly congruent with the rrs tree. Two clusters gathered both Betaprotobacteria and Gammaproteobacteria, suggesting extensive horizontal transfers of acdS, noticeably between plant-associated Proteobacteria.