Caulobacter crescentus 101123 possesses a gene (Mbl1b) encoding a metallo-beta-lactamase with 32% amino acid identity to the L1 metallo-beta-lactamase from Stenotrophomonas maltophilia. The gene was cloned into an expression vector and the enzyme, Mbl1b, was expressed in Escherichia coli. Mbl1b was purified. Catalytic properties for several antibiotics were determined. The enzyme exhibits Michaelis-Menten kinetics for imipenem, meropenem and nitrocefin but substrate inhibition kinetics with cefoxitin, cefaloridine, penicillin G and ampicillin. A homology model predicts Mbl1b has the same structural fold as other metallo-beta-lactamases with a detailed structure very similar to L1 but whereas L1 is a homotetramer, Mbl1b is monomeric. The main differences between Mbl1 and L1 are in the N-terminal region.