Aztreonam-resistant Klebsiella oxytoca strain SL7811 was selected on agar containing 1 microgram of aztreonam per ml from a susceptible strain SL781. The MICs for the resistant mutant towards penicillins, aztreonam and ceftriaxone were much higher, to cefotaxime slightly higher and to ceftazidime unchanged. Synthesis of beta-lactamase was 223-fold greater in the mutant compared with the susceptible strain. SL781 and its resistant mutant SL7811 produced beta-lactamase with the same isoelectric point and substrate profile. The beta-lactamase genes from SL781 and SL7811 were cloned in plasmid pBGS18 giving pBOF-1 and pBOF-4 respectively. The sequences of the two putative promoters indicated two modifications in the resistant plasmid pBOF-4: a transversion (G-->T) in the first base of the -10 consensus sequence and a deletion of one C residue four base pairs upstream of the -10 hexamer.