An investigation was made into the occurrence and biodiversity of Geodermatophilaceae on 78 samples of altered stone surfaces from 24 monuments and natural stones in the Mediterranean basin; it was found that the total microbial counts ranged between 0 and 10(7) cfu g(-1) dry weight. Members of the Geodermatophilaceae family were isolated from 22 of the 78 samples examined, with the incidence of Geodermatophilaceae colonies in the cultivable population ranging from 1% to 100%. The highest percentage was found in six samples of markedly deteriorated stone. Sixty-five strains randomly isolated from the plates were clustered in six different groups by amplified 16S rDNA restriction analysis (ARDRA) using five different restriction enzymes. Twenty-five strains, representing all the ARDRA haplotypes, were characterized further by partial sequencing (350-550 bp) of the 16S rDNA and by analysing 76 morphological, metabolic and physiological properties. The strains were associated with three well-separated clusters of the genera Geodermatophilus, Blastococcus and Modestobacter. On the basis of 16S rDNA sequence and ARDRA analysis, only two strains were found to be related to the two reference strains of Geodermatophilus. All the others could be grouped with Blastococcus aggregatus (19 strains) or the Antarctic species Modestobacter multiseptatus (44 strains), suggesting that it is these two groups, rather than Geodermatophilus, that tend to colonize the stone surfaces, and that Modestobacter-like strains are also found in temperate/Mediterranean climates. From the BOX-polymerase chain reaction (PCR) data, it can be seen that the Modestobacter-like strains, belonging to the most represented ARDRA haplotype (haplotype B, 34 strains), are very polymorphic and that, over a stone surface, there is a wide genetic diversity at the microsite level.