A study was made of the microbial deterioration of frescoes at the monastery of Santa Maria de la Rabida in Huelva, Spain. Main deterioration signs are efflorescence, dark stains and scaling of the surface layer of the picture. Efflorescence is calcium sulphate (gypsum) and minor amounts of calcium chloride, sodium chloride, potassium chloride, nitrates and silicates. The microbial population is mainly composed of the fungi Cladosporium sphaero spermum, Engyodontium album and Aspergillus versicolor and the bacterium Micrococcus luteus. A predatory mite, Tyrophagus palmarum was also found on the microorganisms. A close link exists between the biodeterioration of frescoes and environmental pollution, although wall humidity can be considered the starting point of deterioration.