PDT of T. rubrum. Fungal skin infections caused by T. rubrum are worldwide one of the most common infectious diseases in humans. T. rubrum is clinicaly the most frequently isolated fungl strain. Currently, therapeutic options have insufficient efficacy to kill the fungal spores as well within one single treatment. Photodynamic treatment using the cationic porphyrin Sylsens B in combination with red light kills both the fungal mycelium and its spores within one single treatment. In comparison with the current treatment modalities this is new. This thesis describes the conditions and mechanisms responsible for the successful single photodynamic treatment of T. rubrum. The proven Sylsens B efficacy with red light offers a good perspective for treatment of the fungal nail infection, onychomycoses.