The morphological expression of keratinolysis in fungi isolated from the air of Torino (98 isolates belonging to 36 species) was studied. Light microscopy on whole material and on semithin sections, as well as scanning electron microscopy was used. There were 19 keratinolytically active species, with seven in the genus Chrysosporium (C. indicum, C. keratinophilum, C. pannicola, C. tropicum, C. an. Arthroderma cuniculi, C. an. Pectinotrichum llanense, C. an. Renispora flavissima), four in the genus Malbranchea (M. arcuata, M. fulva, M. sulphurea, M. st. Uncinocarpus reesii), and three in the genus Trichophyton (T. mentagrophytes, T. rubrum, T. terrestre). In addition there were Aphanoascus fulvescens, Beauveria bassiana, Geomyces pannorum v. pannorum, Gymnoascus umbrinus and Myceliophthora vellerea. Most of these species were capable of developing structures related to surface erosion and radial penetration contemporaneously. However Gymnoascus umbrinus, Myceliophthora vellerea, an isolate of C. indicum, C. tropicum and Trichophyton mentagrophytes demonstrated only surface erosion. Different isolates of one species can vary in their production of invasive structures and in degree of keratinolytic activity. Thus such activity, like many biochemical activities of fungi, does not appear to be a constant or rigorously species-specific character.