In view of their outstanding antimicrobial activity, the β-lactam antibiotics have attracted great interest in recent years. In the present communication we report the structure of two unique monocyclic β-lactam antibiotics, nocardicin A (1) and B (2), which are structurally and biologically related to the penicillins and cephalosporins. Nocardicin A (1) was isolated as a major component from a strain of Nocardia. Chemical modification, ¹H NMR analysis, degradation reactions, and stereochemical studies established the structure of nocardicin A (1) and its stereoisomer nocardicin B (2) at the oxime function. The oxime configuration of nocardicin A was shown to be syn to the acylamino group, which is favored for antimicrobial activity. Nocardicin A is active against a variety of gram-negative bacteria, especially Pseudomonas, while nocardicin B is weaker. These antibiotics are unique in being the first potent monocyclic β-lactam antibiotics, containing an oxime function, p-hydroxyphenylglycine units, having a stereochemical relation to penicillins, and acting as enzyme inhibitors in the cell wall biosynthesis of bacteria.