Ten phytotoxic compounds were obtained from a culture of Bipolaris cynodontis cynA, a fungal pathogen of Bermuda grass. The purified compounds were structurally characterized and shown to be a series of derivatives of a quinone compound which has been referred to as cochlioquinone. Of these 10 compounds, 6 have not been described before. All the obtained compounds inhibited the root growth of gramineous plants, such as Italian ryegrass and rice plants, at varying potencies, depending on their structures. In addition, these cochlioquinone derivatives inhibited electron transfer in the mitochondrial respiration system. The molecular hydrophobicity of the cochlioquinone derivatives appears to be an important factor in determining the activity of this class of compounds. © 1998, Pesticide Science Society of Japan. All rights reserved.