Salmonella typhimurium is a potentially intracellular pathogen and is responsible for thousands of reported cases of acute gastroenteritis and diarrhea each year. Although many successful physiological and genetic approaches have been taken to conclude the key virulence determinants encoded by this organism, the total number of uncharacterized reading frames observed within the S. typhimurium genome suggests that many virulence factors remain to be discovered. This study was conducted to evaluate the role of heat induced phage shock protein A (PspA), in the pathogenicity of S. typhimurium. The stress proteins detected on sodium dodecyl sulfate-polyacrylamide gel electrophoresis were identified specifically by immunoblotting with polyclonal antibody against PspA. PspA was produced in response to heat stress at 45A degrees C and it was over-expressed at 65A degrees C. At this temperature, the stressed bacterial cells producing PspA were more virulent (16 folds greater) to female 6-8 week-old Balb/c mice. Correspondency between decrease in LD50 and increase in PspA production during heat stress and lower pathogenicity in non-producing cells that emerged during stress at 55A degrees C represents PspA as an important virulence factor in heat stressed S. typhimurium.