The effect of UV-light on Pityrosporum yeasts (P.) was studied: P. yeasts cultured from the skin were spread on Dixon plates and irradiated with different UVB-and UVA-light dosages and read after three days, controls were not irradiated. Also P. yeasts, immediately after irradiation, were isolated from the plates and studied with an electron microscope. A significant growth inhibition or no growth at all was seen after 25, 50, 75 J/cm2 UVA and 900 mJ/cm2 UVB, a moderate inhibition after irradiation with 250 mJ/cm2 UVB. The growth inhibition was paralleled by ultrastructural degenerative alterations: clumping of ribosomes and lysis of nuclei. The amount of "stacked material" in the vacuoles was diminished or they were completely empty, the cell wall remained unchanged. Our results imply that the positive effect of sunlight on seborrhoeic dermatitis may well be explained by the direct influence of UV-light on the P. yeasts.