N-(3-Carboxy-6-methylphenyl)-3-fluoroazetidin-2-one and a series of related N-aryl-3-halo- and -3,3-dihaloazetidinones 3, in which the halo substituent is a fluorine or a bromine atom, were prepared by using the Wasserman procedure of cyclization of beta-bromopropionamides as a key step. Their affinities for the TEM-1 beta-lactamase were determined and compared with those of a series of tricyclic azetidinones