Thermal tolerance of acid-adapted Escherichia coli O157:H7 or Salmonella in ground beef was evaluated during storage at 4°C or –20°C. Both pathogens were adapted to acidic conditions (pH ∼4.6) by growing in tryptic soy broth supplemented with 1% glucose. A five-strain cocktail of E. coli O157:H7 or Salmonella was grown separately in TSB (pH ∼6.6) and TSB + 1% glucose for 24 h at 37°C to provide cells with or without acid adaptation. Irradiated ground beef was inoculated with either acid-adapted or non-adapted E. coli O157:H7 or Salmonella; the samples stored at 4°C were subjected to heat treatment at 62°C or 65°C on days 1, 7, 14, 21, and 28, and the samples stored at –20°C were subjected to heat treatment at 62°C or 65°C on days 1, 30, 60, 90, and 120. Decimal reduction time (D values) of the pathogens was determined as an indicator of thermal tolerance. Significantly higher D₆₂ values were observed on days 21 and 28 for non-adapted E. coli O157:H7 stored at 4°C and on days 90 and 120 for non-adapted E. coli O157:H7 stored at –20°C (P < 0.05). Higher D₆₂ values were observed on days 21 and 28 among non-adapted Salmonella strains stored at 4°C and on day 28 for acid-adapted strains of Salmonella stored at 4°C (P < 0.05). Higher D₆₂ values for acid-adapted strains of Salmonella stored at –20°C were observed on days 30, 60, and 90 (P < 0.05), when while no differences were observed in the D₆₅ values of acid-adapted and non-adapted strains of E. coli O157:H7 and Salmonella throughout storage at both temperatures (P > 0.05). This suggests that acid adaptation of foodborne pathogens provides a certain level of protection against heat treatment at lower cooking temperatures, while at higher temperatures there were no observed differences between the sensitivity of acid-adapted and non-adapted strains in an actual food system over an extended period of refrigerated and frozen storage.