The heat shock response and thermotolerance of Yersinia enterocolitica in BHI broth and ground pork was studied. Further, the survival and growth of the heat-shocked cells following heat treatment at 55° C and stored at 4 or 25° C under air, vacuum, or modified atmosphere was also examined. Lastly, the irradiation sensitivity of the heat-shocked cells when suspended in pork and packaged either under air or vacuum was determined. As part of the research, virulence of the survivors was examined following heat treatment or irradiation;Exposure of Y. enterocolitica to 45° C for 60 mins. in BHI broth resulted in the heat shock response with increased thermotolerant cells. D[subscript]55 values for heat-shocked and nonheat-shocked controls in BHI broth were 7.7 and 2.0 mins. respectively. Similarly, the D[subscript]55 values in ground pork were 15.6 and 6.5 mins. respectively. Two heat shock proteins were isolated with molecular weights of 70.5 and 58.0 kDa. During storage, the heat-shocked Yersinia grew rapidly under both temperatures and all the atmospheres. There was no significant difference (p > 0.05) in growth rates between the heat-shocked and controls. The heat-shocked cells in ground pork did not exhibit increased resistance when irradiated either under air or vacuum. An average D value of 0.15 kGy was calculated for both the heat-shocked and control cells and under air or vacuum. The surviving cells showed virulent characteristics following either heat treatment or irradiation.