The degradation "in vitro" of human hair, invaded by the dermatophyte Trichophyton mentagrophytes was studied by means of scanning and transmission electron microscopy. The digestion of the hair started as well in the cuticle as in the cortex, in the regions with low content of keratin, especially in the intercellular spaces. The exocuticle and epicuticle were the most resistant parts of the cuticle in fungal degradation, due to its high content in cystine. In the cortex the macrofibrils were first separated from each other and later attacked on the surface and the interior, arriving to be almost completely degraded. At the end only pigmented granules and rests of membranes could be observed. After a month the hair was almost entirely destroyed probably due to an enzymatic process although some mechanical alteration could also be observed.