A novel primase inhibitor, Sch 642305 (1), was isolated from the fermentation broth of the fungal culture Penicillium verrucosum. The structure of 1 was elucidated on the basis of MS and NMR spectroscopic data as a new and unusual bicyclic 10-membered macrolide. The absolute configuration of the asymmetric centers was determined by X-ray crystallographic analysis of the p-bromobenzoate derivative (3). Compound 1 exhibited inhibitory activity against bacterial DNA primase enzyme with an EC(50) of 70 microM.