Thirteen Ralstonia pickettii-like isolates from the environment (water, soil and activated sludge) and human clinical samples (including respiratory secretions of cystic fibrosis patients) were investigated in a polyphasic taxonomic study that employed 16S rDNA sequence analysis, DNA-DNA hybridization, determination of DNA base composition, whole-cell protein analysis, biochemical characterization and PCR-based assays. All isolates were classified as a novel Ralstonia species, for which the name Ralstonia insidiosa sp. nov. is proposed. The type strain, LMG 21421T (= CCUG 46789T), was isolated from the sputum of a patient with acute lymphoblastic leukaemia. R. insidiosa can be differentiated from other species of the genus Ralstonia and phenotypically similar species (including the Burkholderia cepacia complex and Achromobacter xylosoxidans) by a variety of biochemical tests, whole-cell protein analysis and several PCR-based assays. Some outstanding issues in the taxonomy of the genus Ralstonia are also discussed.