A polyphasic taxonomic study (employing whole-cell protein and fatty acid analyses, 16S rDNA sequencing, DNA-DNA hybridization, determination of DNA G+C content, antibiotic susceptibility testing and extensive phenotypic characterization) was performed on 10 isolates that appeared to be related to Alcaligenes faecalis. The isolates were recovered from diverse environments that included human clinical samples. 16S rDNA sequence analysis indicated that these isolates belonged to the genus ACHROMOBACTER: Whole-cell protein analysis distinguished two groups, which were confirmed by DNA-DNA hybridization. Based on the results of this study, the organisms were classified as two novel Achromobacter species, Achromobacter insolitus sp. nov. (type strain, LMG 6003(T)) and Achromobacter spanius sp. nov. (type strain, LMG 5911(T)). Achromobacter insolitus can be distinguished from Achromobacter spanius by its ability to grow on acetamide and to assimilate mesaconate and aconitate, and by its inability to assimilate diaminobutane. Various tests allow the differentiation of both novel species from other Achromobacter species, including growth on acetamide, denitrification and assimilation of D-glucose, D-xylose, mesaconate, aconitate and diaminobutane.