The mechanism of quinolone resistance in 42 food-borne strains of Salmonella typhimurium which showed nalidixic acid resistance was investigated. Analysis of outer membrane proteins and lipopolysaccharides from both quinolone-susceptible and resistant did not shown any differences among the clinical isolates tested. A 285 bp PCR fragment containing the quinolone resistance determining region of the gyrA gene and a 237 bp PCR fragment containing the region of parc homologous to the quinolone- resistance determining region of the gyrA gene, were sequenced. All the resistant strains presented a change at Ser-83 to Phe in the GyrA protein, but no changes were observed in the ParC protein. These findings indicate that this mutation in gyrA gene plays a major role in the acquisition of nalidixic acid resistance in clinical isolates of S. typhimurium.