Strains isolated independently from two patients could be recognized as Ralstonia pickettii biovar 3/'thomasii'. The 16S rDNA sequences of these strains and two other strains of R. pickettii biovar 3/'thomasii' clustered at less than 98% similarity versus all other described Ralstonia species and at less than 97 % versus the two other R. pickettii biovars. The separate species status of R. pickettii biovar 3/'thomasii' was confirmed by DNA-DNA hybridization, indicating less than 60% DNA homology with the R. pickettii biovars Va-1 and Va-2 and with two as-yet unclassified but biochemically similar Ralstonia strains. Phenotypically, this Ralstonia species can be distinguished from all described Ralstonia species by its acidification of D-arabitol and mannitol and by its lack of nitrate reduction and of alkalinization of tartrate and from two as-yet unclassified Ralstonia strains only by its lack of nitrate reduction. The name Ralstonia mannitolytica sp. nov. is proposed, reflecting the characteristic acidification of mannitol. Resistance to desferrioxamine is another difference from R. pickettii and Ralstonia solanacearum. Although several nosocomial outbreaks have been associated with R. mannitolytica, life-threatening infections have not yet been reported, possibly due to misidentification as Pseudomonas fluorescens or Burkholderia cepacia. In at least one of the two cases reported here, the R. mannitolytica isolate was found to be clinically relevant, causing recurrent nosocomial meningitis, with an infected implanted catheter as the source. The type strain of R. mannitolytica is NCIMB 10805T (= LMG 6866T), which was isolated during the first described outbreak as 'Pseudomonas thomasii' at St Thomas' Hospital, London, UK, in 1971.