Pachystermine-A (I) and -B (II), weakly basic alkaloids isolated from Paehysandra terminalis Sieb. et Zucc. (Buxaceae), were investigated. The structures and stereochemistry of these alkaloids are now assigned to the formula I and II, respectively. They represent a novel type of 3,20-diamino-5α-pregnane alkaloid carrying a four-membered β-lactam ring system. © 1967, The Pharmaceutical Society of Japan. All rights reserved.