Chromosulfine (1), a novel cyclopentachromone sulfide, was isolated from a neomycin-resistant mutant of the marine-derived fungus, Penicillium purpurogenum G59. Its structure including stereochemistry was determined using spectroscopic methods, in particular NMR and electronic CD (ECD) analysis and Mosher's method. HPLC-UV/HPLC-MS analyses demonstrated that 1 was produced only in the mutant by a biosynthetic pathway that is silent in the parent strain and was activated by the introduction of neomycin resistance.