Some of the marble carvings which decorate the facade of the Certosa of Pavia are covered by orange-red stains which were previously found not to be due to changes in the chemical or petrographic composition of stone. A series of microscopical and microbiological analyses were therefore undertaken, to see if the stains were due to the presence of microorganisms. In fact, two bacteria producing a red pigment were isolated from samples of stained marble powdered and innoculated into different culture media. These bacteria are heterotrophs and live in the stone microfissures on a substrate of green unicellular algae. They were identified as <i> Micrococcus sp.</i> and <i> Flavobacterium sp.</i> The pigment produced by them is probably brought to the stone surface by water flow, thus causing the diffuse red coloring. In vitro the bacteria were rapidly killed by the addition of very low concentrations of the biocide Bradophen.