We describe the characterization of a novel CTX-M beta-lactamase from Salmonella enterica. Four S. enterica isolates (three of serotype Westhampton and one of serotype Senftenberg) resistant to extended-spectrum cephalosporins (cefotaxime and ceftazidime) were recovered in 2004 from living cockles in three supermarkets located in distant geographic areas in France, which got their supplies from the same fishery. The isolates were found to produce a novel extended-spectrum beta-lactamase (ESBL) belonging to the CTX-M-1 phylogenetic group and named CTX-M-53. The CTX-M-53 beta-lactamase harbored the substitution Asp240Gly, like the CTX-M-15 enzyme, which is specifically implicated in a higher catalytic efficiency against ceftazidime. The bla(CTX-M-53) gene was located on a mobilizable 11-kb plasmid, pWES-1. The complete sequence of pWES-1 revealed the presence of a novel insertion sequence, ISSen2, and an IS26 element upstream and downstream of the bla(CTX-M-53) gene, respectively; however, transposition assays of the bla(CTX-M-53) gene were unsuccessful. IS26 elements may have contributed to the acquisition of the bla(CTX-M-53) gene. Interestingly, the mobilization module of the pWES-1 plasmid was similar to that of quinolone resistance plasmids (carrying the qnrS2 gene) from aquatic sources. Although belonging to two serotypes differentiated on the basis of the O-antigen structure (E1 or E4 groups), the isolates were found to be genetically indistinguishable by pulsed-field gel electrophoresis. Multilocus sequence typing showed that the isolates of serotype Westhampton had a sequence type, ST14, common among isolates of serotype Senftenberg. This is the first characterization of the CTX-M-53 ESBL, which represents an additional ceftazidime-hydrolyzing CTX-M enzyme.