The gene coding for the class A beta-lactamase of Citrobacter diversus has been cloned and sequenced. It contains the information for a 294-amino-acid precursor protein, including a 27-residue N-terminal signal peptide. The deduced sequence of the N-terminal portion of the mature protein is in excellent agreement with that determined by microsequencing of the protein and readily explains the pI differences observed between the naturally occurring forms I and II of the enzyme. The sequence of the mature protein exhibits a very high degree of similarity with that of the Klebsiella oxytoca class A beta-lactamase.