Three prenylated indole alkaloids (1–3), including two new penicimutamides D–E (1–2), were isolated from a diethyl sulfate mutant of the marine-derived fungus Penicillium purpurogenum G59. The structures of 1–3, and their absolute configurations, were determined by spectroscopic methods, including X-ray crystallography and CD analyses. HPLC-UV and HPLC-MS analyses showing that 1–3 were only produced in the mutant evidenced that the silent biosynthetic pathways that produce 1–3 in the parental strain are activated by DES mutagenesis.