Scanning electron microscopy confirmed the presence of bacteria and biofilm on all substrates, in situ and in the laboratory. Bacterial populations were not favored on substrates of a particular mineralogy in situ or in the laboratory; however, sheltered sites favored the development of a rich, homogeneous biofilm. The presence of salts had a profound effect on the appearance of the biofilm, in situ and in the laboratory experiments, causing contraction, coagulation, and desiccation. Weathering experiments also indicated that salts could also interact destructively with filamentous bacteria, in particular with the genus Streptomyces, and cause severe damage to stone. This biodeterioration could be attributed to biofilm production, which may enhance the effects of salt crystallization pressure in the pore system of the stone. The extent of FDA hydrolysis, which is an index of growth, was strongly correlated to changes in stone properties. The additional mechanical effects of actinomycete hyphal penetration on the stone were clearly evident from the SEM analysis. Similar reactions to salts are thus observed by SEM in situ and in the laboratory, and these can be linked to patterns of growth and serious damage to stone.