Before the present restoration, the wall paintings of St. Christopher were in a bad state of conservation, with a diffused white veiling, with a superficial scaling of the plaster. The chemical analyses, the SEM observations, the thermohygrometric measurements, and the humidity tests of the walls are reported, together with a short description of the restoration techniques. The black crusts in the brick walls, analyzed with biological methods, contain a green alga and five different species of fungi. The <i>Amobasidium pullulans</i>, with black spores, color the bricks. Two fungicides tested (Metatin 58-10 and Metatin 101) suppress the growth of microorganisms. The application of fungicides on the brick walls three times a year is suggested to prevent microbial growth. -- AATA