The P. aeruginosa clinical isolates with multiple antibiotic resistance from Siriraj Hospital showed a region of closely related integron elements and transposons which carry at least 10 antibiotic resistance genes. The resistance genes from PaTh2 have been directly cloned and the genes have been sequenced within the total length of 26 kb. Resistance to aminoglycosides was conferred by three different aminoglycoside transferase genes, aadA2, aph, and aadB. Resistance to β-lactams was conferred by three different β-lactamases, blaPSE-1, blaCEF-1 (also called blaVEB-1), and blaOXA-10. Rifampin resistance was conferred by the arr-2 gene which encoded for a ribosylating transferase. Resistance to chloramphenicol was conferred by an active transport protein encoded by the cmlA gene. The resistance genes to quaternary ammonium compound and sulfonamide were also identified, qacEΔ1 and sul1, respectively. The arr-2 gene has been further analyzed to confirm rifampin resistance in laboratory strains of E. coli and P. aeruginosa. The CEF-1 β-lactamase has been purified for kinetic studies. The oxy-imino cephalosporins were good substrates for CEF-1 β-lactamase, whereas ampicillin was the poorest substrate. Most antibiotic resistance genes were located on gene cassettes as parts of two integron elements, and some were not cassettes but were connected to this region by unknown mechanisms of recombination. The adjacent transposon genes indicated the possibility of spread of this large collection of antibiotic resistance genes among other bacteria as seen in 4 other clinical isolates of P. aeruginosa. The sequence adjacent to the resistance cluster showed no homology to any sequence in the GenBank implying the cluster was not chromosomal in origin.