Aeromonas species rarely cause pulmonary infection. We report, for what is believed to be the first time, a case of severe pneumonia in a cancer patient caused by Aeromonas caviae. Detailed microbiological investigation revealed that this isolate carried three beta-lactamase-encoding genes (encoding MOX-4, CTX-M-3 and TEM-1) conferring resistance to all beta-lactams but imipenem. The beta-lactamase with a pI of 9.0 was transferred by conjugation and associated with a 7.3 kb plasmid, as demonstrated by Southern blot hybridization. Analysis of the nucleotide and amino acid sequences showed a new ampC gene that was closely related to those encoding the MOX-1, MOX-2 and MOX-3 beta-lactamases. This new plasmid-mediated AmpC beta-lactamase from China was named MOX-4. This is believed to be the first report of MOX-4, CTX-M-3 and TEM-1 beta-lactamases in a multiresistant A. caviae.