D-values of acid adapted Escherichia coli O157:H7 and Salmonella were determined in meat serum and ground beef stored at 4 and -20°C as an indicator of thermal tolerance. Pathogens were acid adapted by growing in Tryptic Soy Broth with 1% glucose (TSB+1%G). Five-strain cocktail of both bacteria were grown and inoculated meat serum was heated to 58, 62, and 65°C, while inoculated ground beef was heated to 62 and 65°C on day 1, 7, 14, 21, and 28 (4°C), and on day 1, 30, 60, 90, and 120 (-20°C). Higher (P<0.05) D-values were observed for acid adapted E. coli O157:H7 in meat serum at 58, 62, and 65°C; and for acid adapted Salmonella at 58 and 62°C, but no difference (P>0.05) was observed at 65°C. In ground beef D62-values of non-adapted E. coli O157:H7 were higher (P<0.05) on day 21 and 28 (4°C) and on day 90 and 120 (-20°C). Higher (P<0.05) D62-values were observed on day 21 and 28 at 4°C amongst non-adapted strains and on day 28 for acid adapted strains of Salmonella. Higher (P<0.05) D62-values of acid adapted Salmonella were observed on day 30, 60, and 90 when stored at -20°C while no differences (P>0.05) were observed in D65-values of acid adapted and non-adapted strains throughout storage (4 and -20°C);Toxicity of acid adapted E. coli O157:H7 to African Green Monkey kidney cells (Vero cells) was also determined. Vero cells were grown separately and subjected to toxin produced by acid adapted and non-adapted E. coli O157:H7 grown in laboratory media and meat serum. Comparison of toxicity was made after storing samples for 1 and 7 days at 4 and -20°C. No differences (P>0.05) were observed between the toxicity of acid adapted and non-adapted cells in laboratory media and meat serum at both temperature. Higher (P<0.05) toxicity was observed on day 1 as compared to day 7 at both storage temperatures. Higher (P<0.05) verotoxin production was observed in meat serum inoculated with non-adapted cells on day 1 as compared to acid adapted cells.