On the basis of phenotypic and genotypic characteristics and 16S rRNA gene sequence analysis, novel species belonging to the genera Sphingomonas and Brevundimonas were identified from samples taken from the Russian space laboratory Mir. Strain A1-18(T) was isolated from the air. 16S rDNA sequence analysis showed that strain A1-18(T) formed a coherent cluster with Sphingomonas sanguinis, Sphingomonas parapaucimobilis, Sphingomonas paucimobilis and Sphingomonas roseiflava with sequence similarity of 97.5-98.6 %. Similar to other Sphingomonas species, the G+C content was 66.1 mol%, but DNA-DNA hybridization rates at optimal temperatures among these related species were only 24.7-51.7 %. Strain A1-18(T) can be differentiated biochemically from related species. Strain W1-2B(T) was isolated from condensation water. It forms a distinct lineage within the genus Brevundimonas, forming a coherent cluster with Brevundimonas vesicularis, Brevundimonas aurantiaca and Brevundimonas intermedia. 16S rDNA sequence similarities were 98.6-99.5 % and the G+C content was 66.5 mol%, similar to other Brevundimonas species, but DNA-DNA relatedness was only 50.2-54.8 %. Strain W1-2B(T) also showed some differential biochemical properties from its related species. A series of polyphasic taxonomic studies led to the proposal of two novel species, Sphingomonas yabuuchiae sp. nov. (type strain A1-18(T)=GTC 868(T)=JCM 11416(T)=DSM 14562(T)) and Brevundimonas nasdae sp. nov. (type strain W1-2B(T)=GTC 1043(T)=JCM 11415(T)=DSM 14572(T)).