Microorganisms are considered as important resources for researchers to prove and develop new natural products and have yielded some of the most important products of the pharmaceutical industry, such as penicillin, erythromycin and avermectin.1–4 Although research in antibiotics and natural products has declined significantly during the last decade,5–7 the search for novel bioactive microbial metabolites for potential pharmaceutical applications has been and still is important.8,9 During the course of searching for novel microbe-derived bioactive secondary metabolites, we investigated the chemical constituents of a strain Streptomyces sp. HS-NF-1046. As a result, a new polysubstituted cyclopentene derivative, named hisunic acid (1, Figure 1), was isolated from the fermentation broth of this strain. In this paper, we describe the fermentation, isolation, structure elucidation and bioactivity of 1.