We study the influence of microscopic funguses and the products of their vital activity (mainly metabolites, which contain organic acids) on the mechanical properties of fiber materials (such as fiberglass and policaproamid fibers). It is found that fungus grown on the fibers bring the fibers to significant change of the ultimate stress and to relative elongation of the materials, while metabolites (organic acids) change the mechanical properties of the fiberglass only. Besides the mechanical properties, the chemical structure of the materials changes. The theoretical results are in good agreement with the results of electronic microscopy.