Two bacterial strains, CP173-2(T) and CP1D(T), were isolated from biological soil crusts (BSCs) collected in the Colorado Plateau, USA. Both strains were pigmented, Gram-negative, non-motile rods and produced abundant mucus. They contained C(16 : 0), C(18 : 1)omega7c and C(14 : 0) 2-OH as the predominant cellular fatty acids, ubiquinone-10 as the isoprenoid quinone and sphingoglycolipid. Based on the above characteristics, the isolates were assigned to the family Sphingomonadaceae; 16 rRNA gene signature nucleotides placed them within the genus Sphingomonas. Strains CP173-2(T) and CP1D(T) had a 16S rRNA gene sequence similarity of 96.7 % with each other and 91.6-98.9 % sequence similarity with other species in the genus, indicating that they represent two separate, and possibly novel, species. The closest species to strains CP173-2(T) and CP1D(T) were, respectively, Sphingomonas dokdonensis (98.9 % gene sequence similarity) and Sphingomonas panni (97.9 %). However, strain CP173-2(T) exhibited a DNA-DNA relatedness of only 32.5 % with the type strain of S. dokdonensis. Similarly, the DNA-DNA relatedness between strain CP1D(T) and the type strain of S. panni was only 18 %. Phenotypic characterization supported this low relatedness. On the basis of this evidence, we propose that the new strains represent two novel species, for which the names Sphingomonas mucosissima sp. nov. (with type strain CP173-2(T)=ATCC BAA-1239(T)=DSM 17494(T)) and Sphingomonas desiccabilis sp. nov. (with type strain CP1D(T)=ATCC BAA-1041(T)=DSM 16792(T)) are proposed.