During the course of our studies on new antibiotics, we found that Trichoderma sp. Tk-1 produced substances A and B which showed inhibitory activities against both Staphylococcus aureus and Escherichia coli. From their chemical and physical properties, substances A and B were identified as trichoviridin1) and dermadin2), respectively. The strain Tk-1 was obtained from a soil sample and classified in the Trichoderma koningiigroup according to RIFAI3). This paper presents some characteristics of the strain, the isolation of substances A and B, and the approach to the structure elucidation of trichoviridin.