Several nocardioform actinomycetes were isolated from tufaceous surfaces with whitish-grey patinas in the catacombs of St Callistus in Rome, Italy. The morphology of the isolates and their chemotaxonomic characteristics such as LL-diaminopimelic acid in the cell-wall peptidoglycan, the major menaquinone of MK-9(H(4)), phosphatidylinositol, phosphatidylcholine, phosphatidylglycerol and diphosphatidylglycerol as the major polar lipids, as well as complex cellular fatty acid patterns with anteiso-C(15:0), iso-C(16:0) and iso-C(15:0) as predominating components, were in agreement with their classification as members of the genus Kribbella by 16S rRNA gene sequence analysis. The isolates fell into two clusters as revealed by their ribosomal intergenic spacer, RiboPrint and cellular fatty acid patterns and by their MALDI-TOF mass spectra. The two clusters were represented by the strains BC631(T) and BC633(T) which shared 97.9% 16S rRNA gene sequence similarity. Strain BC631(T) represented a cluster of yellow pigmented strains and was a phylogenetic neighbour of Kribbella koreensis DSM 17837(T) (gene sequence similarity 98.0%), while strain BC633(T) was related to Kribbella flavida DSM 17836(T) and Kribbella karoonensis DSM 17344(T) (gene sequence similarities of 98.8% and 98.6%, respectively). Strains BC631(T) and BC633(T) could be differentiated from each other and from their closest phylogenetic neighbours by phenotypic characteristics and DNA-DNA relatedness values far below 70%. It is concluded that the two new strains represent two novel species, for which the names Kribbella catacumbae sp. nov. (type strain BC631(T)=DSM 19601(T)=JCM 14968(T)) and Kribbella sancticallisti sp. nov. (type strain BC633(T)=DSM 19602(T)=JCM 14969(T)) are proposed.