Screening of microbial fermentation broths for herbicidal activity led to the discovery of a novel polyketide, herboxidiene, from an actinomycete identified as a member of the Streptomyces chromofuscus cluster. A 14- to 20-fold increase in fermentation production of herboxidiene was achieved as a result of media optimization. Herboxidiene was purified using successive reverse phase C18 steps and Sephadex LH-20 chromatography. Its molecular formula, C25H42O6, was determined by HRFAB-MS. Herboxidiene demonstrated exceptionally potent, selective, herbicidal activity against a variety of weed species and was inactive against wheat, even at rates as high as 5.6 kg/hectare.