When Trichophyton mentagrophytes thalli are placed on a medium containing 300 microgram/ml of coumarin, their growth rate is drastically reduced and the newly formed mycelium consists of curled and branched hyphae showing subapical bulges and swollen tips. Under the electron microscope, the most relevant abnormalities concern the cell walls which are often thickened by aberrantly shaped zones that are usually smeared unevenly over the surface of the "primary" wall instead of being deposited in discrete ridges. An ultracytochemical analysis suggests that the irregular thickenings are formed of chitin. Arguments suitable to explain the phenomena observed are discussed.