Two novel isoquinoline fungal derivatives, pyrenoline-A and -B, were isolated from the culture fluid of Pyrenophora teres, a pathogen of barley, and characterized by spectroscopic and X-ray diffraction techniques. Pyrenophora teres, the causative fungus of net blotch disease, is an important pathogen of barley (Hordeum vulgare). Net blotch disease occurs wherever barley is grown in temperate and humid regions of the world.1 Due to the increasing popularity of barley, diseases such as net blotch have ever greater significance.2 Several groups have previously isolated bioactive compounds from P. teres.3,4 Smedegard-Peterson isolated aspergillomarasmine-A and -B.5 The aspergillomarasmines were first described by Haenni and coworkers as metabolites of Aspergillus oryzae and A. flavus.6 They have also been isolated from other plant pathogens including Fusarium oxysporum f sp melonis and Colletotrichum gloeosporoides.8,9 Pyrenolide-A, a ten-membered macrolide, has also been reported from P. teres.3 In the present study we report the isolation, structure characterization, and biological activity of two novel phytotoxic isoquinoline derivatives, pyrenoline-A (1) and -B (2).