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The aim of the present work is to point out the role of microorganisms in biodeterioration from two different historical monuments located in Romania: the refectory of the Hurezi monastic complex and Amărăşti church. Mural painting was made in al frescoes technique applied on the brick or on the wooden poles. HOBO LCD data loggers revealed values of temperature between 5 and 24° and relative humidity between 60 and 90% which are favorable for the colonization by bacteria and fungi. Two different genera of moderately halophilic bacteria (Nesterenkonia and Halobacillus) and 5 different genera of fungi (Antrodia, Aspergillus, Alternaria, Penicillium and Ulocladium) had been identified either by molecular techniques or by cultural and mycelium characteristics. They developed a pink and black pigmentation. The main vehicles for spreading of microorganisms are as follows: insects, condensation, convection currents and human activity. Bacterial cells and mycelia have been found both on the surface of the mural paintings, mortars, wooden structure as well as in the fissures and pores. Bacteria producing carotenoids grow on the surface of the substrates but fungi can be located both on the substrate and under it developing in the same time aerial and substrate hyphae. Mycelia produce the peeling and loosing of the pictorial layer. Organic acids act as chelating agents and mycelium as pulling agent. The aesthetical and structural changes of mural painting can be prevented by monitoring the environment and avoiding contamination. If damages took place, understanding their origin and proper treatment will contribute to preserving of cultural heritage. [ABSTRACT FROM AUTHOR]