High doses of ultraviolet radiation (UVR) have well-known inhibitory effects upon dermatophytes. In the present study, the effect of repetitive low doses of UVR on mycelial growth of dermatophytes was tested. Pellets of Trichophyton rubrum, T. mentagrophytes and Microsporum canis were placed between two thin layers of Sabouraud glucose agar. Obverse, reverse or both sides of these 'sandwich' agars were irradiated for 10 days twice daily with 0.13 or 0.17 J cm-2 UVB. To simulate microaerophilic conditions, one or both agar sides were covered by transparent airtight plastic lids. In addition, T. rubrum was also grown as usual on plates of Sabouraud glucose agar without any covering, and irradiated on its obverse side twice daily with UVA (13.5 J cm-2), UVB (0.17, 0.34 or 0.69 J cm-2) or infrared light, or once only with 3.8-15.1 J cm-2 UVB. As a result, thallus diameters of all strains were found to be reduced by repetitive UVB irradiation under both aerobic and microaerophilic growth conditions. T. rubrum was unaffected by infrared irradiation, responded with an increased pigmentation to UVA (13.5 J cm-2 twice daily) and was inhibited by a single dose of 15.1 J cm-2 UVB. Negative phototropism of dermatophytes is a new observation. It may be biologically relevant as a mechanism to evade harmful doses of UVR.