Salmonella is an important cause of foodborne illness; however, identifying the source of these infections can be difficult. This is especially true for Salmonella serotype Typhimurium, which is found in diverse agricultural niches. Extended-spectrum cephalosporins (ESC) are one of the primary treatment choices for complicated Salmonella infections. In Salmonella, ESC resistance in the United States is mainly mediated by blaCMY genes carried on various plasmids. In this study, we examined whether the characterization of blaCMY plasmids, along with additional information, can help us identify potential sources of infection by Salmonella, and used serotype Typhimurium as a model. In the United States, monitoring of retail meat, food animals, and ill persons for antimicrobial-resistant Salmonella is conducted by the National Antimicrobial Resistance Monitoring System. In 2008, 70 isolates (70/581; 12.0%) (34 isolates from retail meat, 23 food animal, and 13 human) were resistant to ceftriaxone and amoxicillin/clavulanic acid. All were polymerase chain reaction (PCR)-positive for blaCMY and 59/70 (84.3%) of these genes were plasmid encoded. PCR-based replicon typing identified 42/59 (71.2%) IncI1-blaCMY plasmids and 17/59 (28.8%) IncA/C-blaCMY plasmids. Isolates from chickens or chicken products with blaCMY plasmids primarily had IncI1-blaCMY plasmids (37/40; 92.5%), while all isolates from cattle had IncA/C-blaCMY plasmids. Isolates from humans had either IncA/C- blaCMY (n=8/12; [66.7%]) or IncI1- blaCMY (n=4/12 [33.3%]) plasmids. All of the IncI1-blaCMY plasmids were ST12 or were closely related to ST12. Antimicrobial susceptibility patterns (AST) and pulsed-field gel electrophoresis (PFGE) patterns of the isolates were also compared and differences were identified between isolate sources. When the source of a Typhimurium outbreak or sporadic illness is unknown, characterizing the outbreak isolate's blaCMY plasmids, AST, and PFGE patterns may help identify it.