A study of the presence of cyanobacteria and algae in different building materials from Spain (Salamanca, Seville and Toledo Cathedrals) and Sweden (Lund Cathedral) and their possible relation to the stone decay was accomplished. Colonization of stone with the cyanobacterium Microcoleus vaginatus and the chlorophyte Klebsormidium flaccidum was also induced in the laboratory. In both field and laboratory samples, the microbial film, spontaneously detached, showed on its reverse side the presence of grains removed from the stone surface, thus causing mechanical deterioration on the colonized materials.