Four pseudomonad strains 51Men, 54Pin, 62Car and 65Phen were recently isolated on the monoterpenes (+)-menthene, alpha-pinene, 2-carene and alpha-phellandrene as sole carbon source and nitrate as electron acceptor. These bacteria were characterised. The motile, mesophilic, Gram-negative rods had a strictly respiratory metabolism. Monoterpenes as carbon sources were completely mineralised to carbon dioxide. The physiology of all strains was very similar, but displayed an individual utilisation preference for the isolation substrate. The fatty acid composition of whole cells showed a high degree of similarity to that of Alcaligenes faecalis. Comparative 16S rDNA data analysis placed the isolates into the beta-subclass of Proteobacteria in a common offshoot together with Alcaligenes and Bordetella species. On the basis of these characteristics, the strains are described as a new species belonging to the genus Alcaligenes, A. defragrans sp. nov., with strain 54Pin (DSM 12141T) as type strain.