Pulsed-field gel electrophoresis, plasmid profiling, and phage typing were used to characterize and determine possible genetic relationships between 48 Salmonella enterica subsp. enterica isolates of pig origin collected in Catalonia, Spain, from 1998 to 2000. The strains were grouped into 23 multidrug-resistant fljB-lacking S. enterica serovar 4,5,12:i:- isolates, 24 S. enterica serovar Typhimurium isolates, and 1 S. enterica serovar 4,5,12:-:- isolate. After combining the XbaI and BlnI macrorestriction profiles (XB profile), we observed 29 distinct subtypes which were grouped into seven main patterns. All 23 of the 4,5,12:i:- serovar strains and 10 serovar Typhimurium isolates were found to have pattern AR, and similarities of >78% were detected among the subtypes. Three of the serovar Typhimurium DT U302 strains (strains T3, T4, and T8) were included in the same 4,5,12:i:- serovar cluster and shared a plasmid profile (profile I) and a pattern of multidrug resistance (resistance to ampicillin, chloramphenicol, streptomycin, sulfonamide, tetracycline, gentamicin, and trimethoprim-sulfamethoxazole) commonly found in monophasic isolates. This led us to the conclusion that strains of the S. enterica 4,5,12:i:- serovar might have originated from an S. enterica serovar Typhimurium DT U302 strain.