Pyrrolams A to D (1–4), biosynthetically new pyrrolizidinones produced by Streptomyces olivaceus, have been discovered by chemical screening. Their structures have been determined by spectroscopic methods and by a comparison of (R)‐dihydropyrrolam A (5) with a synthetic sample obtained from (S)‐proline. The pyrrolams differ from the necine bases found in plant‐originating pyrrolizidine alkaloids by their less complicated structures. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim