Objectives To characterize the genetic determinants responsible for extended-spectrum cephalosporin (ESC) resistance of <scp>d</scp>-tartrate-positive Salmonella enterica subsp. enterica serovar Paratyphi B (serovar Paratyphi B dT+) strains that have emerged in poultry and humans in Belgium during 2008–10. Methods The ESC resistance genes among non-redundant serovar Paratyphi B dT+ strains were determined using PCR and sequencing. ESC phenotypes were horizontally transferred by conjugation. Extended-spectrum β-lactamase (ESBL)- or AmpC-carrying plasmids were typed by PCR-based replicon typing, plasmid multilocus sequence typing and restriction fragment length polymorphism. The genetic relationship of ESC-resistant strains was assessed by XbaI PFGE and multilocus sequence typing. Results Since 2008, the proportion of serovar Paratyphi B dT+ strains from broiler origin has increased significantly to reach 36.5% in 2010. Among 95 non-duplicate serovar Paratyphi B dT+ strains, 35% were resistant to ESCs. At the same time, a few ESC-resistant serovar Paratyphi B dT+ strains from humans were also detected in Belgium. The most prevalent ESBL gene, bla CTX-M-1 , and the AmpC cephalosporinase gene bla CMY-2 were identified on various conjugative IncI1 plasmids of different sequence types and with different additional non-β-lactam phenotypes. Interestingly, the bla CTX-M-2 gene was located on large multireplicon IncHI2/P plasmids. In addition, highly ESC-resistant strains contained both the ESBL CTX-M-2 and the AmpC CMY-2 encoded by the IncHI2/P and IncI1 plasmids, respectively. All ESC-resistant serovar Paratyphi B dT+ strains belonged to sequence type 28 and showed the common PFGE pattern X8, as well as the chromosomal class 2 integron cassette array dfrA1 - sat2 - aadA1 previously described in the European poultry-associated serovar Paratyphi B dT+ clonal population. Conclusions This study showed that the clonal population of multidrug-resistant serovar Paratyphi B dT+, persisting in broilers in Belgium for ...