Seven psychrotolerant, Gram-negative bacterial strains, five dust- and airborne isolates (MA101b(T), MA306a, MA405/90, MA-olki(T) and NW12(T)) and two from the Antarctic (Ant 20 and M3C203B-B), were subjected to a polyphasic characterization to determine their taxonomic position. High 16S rDNA sequences similarities (99.3-100.0 %) demonstrated that they were closely related to each other. Phylogenetic evaluation of their 16S rDNA sequences revealed that they are members of the genus Sphingomonas sensu stricto, encompassing a separate branch within this genus. They shared 94.4-96.6 % 16S rDNA sequence similarity with species of this genus. All Sphingomonas-specific signature nucleotides were also detected. The presence of the major ubiquinone Q-10, sym-homospermidine as the predominant polyamine, Sphingomonadaceae-specific sphingoglycolipid in the polar lipid patterns and a fatty acid profile containing C(14 : 0) 2-OH and lacking 3-OH fatty acids were in agreement with identification of these strains as members of the genus Sphingomonas sensu stricto. Results from DNA-DNA hybridizations and comparison of protein patterns indicated that the seven strains are members of three distinct species. One species is represented by strains MA101b(T), MA306a and MA405/90, the second by strains NW12(T), Ant 20 and M3C203B-B and the third by one strain, MA-olki(T). Their distinction at the species level was also supported by results of biochemical characterization and partly supported by riboprints and genomic fingerprints. On the basis of these results, three novel species of the genus Sphingomonas are proposed: Sphingomonas aurantiaca sp. nov., consisting of strains MA101b(T) (=DSM 14748(T)=LMG 21377(T)), MA306a and MA405/90 (=DSM 14749=LMG 21378), Sphingomonas faeni sp. nov. MA-olki(T) (=DSM 14747(T)=LMG 21379(T)) and Sphingomonas aerolata sp. nov., represented by strains NW12(T) (=DSM 14746(T)=LMG 21376(T)), Ant 20 (=ICMP 13599) and M3C203B-B (=SMCC M3C203B-B).