A new 16-membered macrolide designated as rhizoxin was isolated as a toxin produced by Rhizopus chinensis, the causal agent of rice seedling blight . The skeletal structure was determined by detailed NMR spectroscopic investigation of this compound and of its derivatives. Rhizoxin induced at a concentration of 10 ng/ml abnormal swelling of rice seedling roots, which is one of the characteristic symptoms of this disease. This compound also exhibited potent antifungal activity but little effect against bacteria.