A polyphasic study was undertaken to establish the taxonomic status of two Modestobacter strains isolated from the surface of deteriorated sandstone of a historic building in Salamanca, Spain. The strains, isolates MDVD1(T) and MON 3.1(T), were found to have chemotaxonomic and morphological properties consistent with their classification in the genus Modestobacter and to form distinct phyletic lines in the Modestobacter 16S rRNA gene tree. Isolate MDVD1(T) was found to be closely related to the type strain of Modestobacter versicolor (98.7 % similarity) and isolate MON 3.1(T) to the type strain of Modestobacter multiseptatus (98.6 % similarity). The isolates were distinguished readily from one another and from the Modestobacter type strains by a broad range of phenotypic properties, by qualitative and quantitative differences in fatty acid profiles and by BOX fingerprint patterns. On the basis of these data, it is proposed that the isolates be classified in the genus Modestobacter as Modestobacter lapidis sp. nov. and Modestobacter muralis sp. nov., with isolates MON 3.1(T) (CECT 8844(T) = DSM 100206(T)) and MDVD1(T) (CECT 8845(T) = DSM 100205(T)) as the respective type strains.