The phenotypic and genotypic characteristics of a historical collection of strains identified as Achromobacter denitrificanswere examined. Sequence analysis of a 765 bp nrdA gene fragment revealed that eight of these strains belonged to the recently described Achromobacter aegrifaciens, Achromobacter mucicolens, and Achromobacter insolitus, and that one strain belonged to Achromobacter xylosoxidans. The analysis also suggested the presence of four novel species of the genus Achromobacter among the remaining strains. The latter was confirmed by multilocus sequence analysis of concatenated nusA, eno, rpoB, gltB, lepA, nuoL andnrdA gene fragments and extensive genotypic and phenotypic characterization. We propose to name these novel species as Achromobacter agilis sp. nov., nom. rev. (type strain LMG 3411T=CCUG 62454T), Achromobacter pestifer sp. nov., nom. rev. (type strain LMG 3431T=CCUG 61959T) , Achromobacter kerstersii sp. nov. (type strain LMG 3441T=CCUG 62449T) and Achromobacter deleyi sp. nov. (type strain LMG 3458T=CCUG 62433T).