An abnormal cell wall component in Trichophyton mentagrophytes treated with Phosfon D: ultrastructural demonstration. – Phosfon D, a growth-retarding chemical in higher plants, at the concentration of 5 × 10−4Minhibits the development of the dermatophytic fungusTrichophyton mentagrophytes. In the treated samples the diameter of the thallus is about 50% smaller than normal. The mycostatic effect of Phosfon D is accompanied by characteristic alterations of the cell wall ultrastructure with the appearance of dense granular masses not revealable after conventional fixation, but only after employing TAPO containing fixatives.