A new phytotoxin, pyridazocidin, was produced in shake-flask cultures of a Streptomyces sp. soil isolate. A combination of solvent partitioning, size-exclusion chromatography, and high-pressure liquid chromatography (HPLC) was used to isolate the material. The structure was assigned based on mass spectroscopy and nuclear magnetic resonance (NMR) experiments. Greenhouse evaluations indicated rapid necrosis and chlorosis of treated weeds, particularly of giant foxtail. Isolated chloroplasts consumed oxygen in the presence of pyridazocidin over a concentration range also inhibiting plant growth. Pyridazocidin represents the first reported natural product that appears to act via reversible oxidation/reduction linked to photosynthetic electron transport.