A novel phytotoxin, exserohilone (1), was isolated from the culture broth of Exserohilum holmii, a pathogenic fungus of the weedy plant Dactyloctenium aegyptium. The structure of exserohilone (1) was elucidated by X-ray diffraction analysis of its bis(p-bromobenzoate) derivative (3). 9,10-Dihydroexserohilone (2) was also isolated, and its structure was determined by spectral methods. Compounds 1 and 2 show nonselectivity toward several plant species at 10⁻⁴-10⁻⁵ M, causing necrotic lesions usually surrounded by a reddish brown border. Monocerin (4) was identified as a major component of the EtOAc extract from the culture.