Tito Bustillo cave (Ribadesella, Spain) contains valuable Palaeolithic paintings, which date back 15 000-20 000 years. Since 1969, the cave has been open to the public. Rock wall surfaces, spelaeothems and soils are covered by apparent biofilms of phototrophic microorganisms, which develop under artificial lighting. In addition, rock surfaces present conspicuous bacterial growth in the form of round colonies of different colours and about 1-2 mm in diameter. Even the famous Paintings Panel shows some evident microbial growth. In the present study, bacterial communities on the paintings and on the rock surfaces near the paintings were analysed by culture-independent techniques, including polymerase chain reaction (PCR) amplification of bacterial 16S rRNA genes (16S rDNA), phylogenetic sequence analyses and genetic community fingerprinting by denaturing gradient gel electrophoresis (DGGE). DGGE fingerprints showed complex bacterial community patterns. Forty-one clones matching DGGE bands of the community fingerprints were sequenced, representing about 39% of DNA fragments in the DGGE patterns. Phylogenetic sequence analyses revealed a high number of phylogenetically novel 16S rDNA sequence types and a high diversity of putatively chemotrophic and heterotrophic bacteria. Sequences were phylogenetically most closely related to the Proteobacteria (20 clones), green non-sulphur bacteria (three clones), Planctomycetales order (one clone), Cytophaga-Flexibacter- Bacteroides division (one clone) and the Actinobacteria (four clones). Furthermore, we report the presence of members of the Acidobacterium division (12 clones) in a karstic hypogean environment. Members of this phylum have not so far been detected in these particular environments.