Examines the problems concerning the <i>Actinomycetes</i> biodeteriogens on frescoes of underground environments. This illustrates: 1) the serious damage suffered by a complete series of famous frescoes; 2) the techniques of nondestructive sampling; and 3) the media used for isolating both on site and in the laboratory. Presents two new aspects: the study of the relation between the microclimate of the environment where the frescoes are situated and the humidity required by <i>Actinomycetes</i>. This has highlighted the close correlation between thermohygrometric values necessary for the development of <i>Actynomycetes</i> and the actual values. The isolated strains are classified for the first time, as those which attack mural paintings. They are <i> Streptomyces rectus flexibilis, S. griseoulus, S. cinereoruber, S. vinaceus, S. albus</i> and <i>Nocardia sp</i>.