A clinical isolate of Proteus mirabilis Lil-1 was obtained from a Vietnamese patient hospitalized in Paris, France. This isolate was resistant to cephalosporins, and there was marked synergy between cephalosporins and clavulanic acid together with unusual synergy between cefoxitin and cefuroxime. PCR analysis revealed the presence of blaVEB-1, an integron-located gene coding for an extended-spectrum beta-lactamase (ESBL) identified previously in an Escherichia coli isolate MG-1 from Vietnam. Using class 1 integron primers and blaVEB-1 intragenic primers, the insert region of the blaVEB-1-containing integron along with flanking sequences were amplified from P. mirabilis Lil-1 whole-cell DNA. A novel class 1 integron, In55, was identified that contained, in addition to intI1, qacEDelta1, sul1 and Orf5 genes, an 8 kb variable region. This region was comparable in size to that found previously in E. coli MG-1, but different from those previously identified in two Pseudomonas aeruginosa isolates from Thailand. In55 was located on a 190 kb self-transferable plasmid, which was different in size and structure from that found in E. coli MG-1. The finding of blaVEB-1 on different plasmids and integrons in enterobacterial isolates underlines the interspecies spread of this novel ESBL gene.