In our search for new antitumor agents, a Streptomyces sp. was found that produces the known antibiotic tomaymycin (1a) and oxotomaymycin (1b). During the course of isolating these two compounds, a third metabolite, PD 125375, was obtained. This new compound is devoid of antitumor and antimicrobial activity. The current interest in tomaymycin, a highly potent antitumor agent, prompted us to examine PD 125375 to determine if it is structurally related to tomaymycin. Through ¹H-¹H correlation (COSY), nuclear Overhauser enhancement correlated (NOESY), ¹H-¹³C heteronuclear correlation (HETCOR) NMR experiments, and single-crystal X-ray diffraction analysis, the structure of PD 125375 was assigned as 2. This structure includes the 3-ethylidinepyrrolidine moiety present in tomaymycin, with a cis relationship between H-10 and H-10a and an E configuration for the exocyclic double bond, which are the same stereochemical features as tomaymycin. The biosynthetic pathway leading to PD 125375 remains obscure.