The impact of growth conditions (anaerobiosis, growth phase, NaCl, pH, and temperature) on the development of acid tolerance in Escherichia coli O157:H7 was investigated directly (DpH1.15) and indirectly by monitoring the specific activity of acid phosphatase. Anaerobic growth of O157:H7 strain 43895 in synthetic rumen fluid resulted in earlier development of acid tolerance than aerobic growth. However, stationary-phase cells of both aerobic and anaerobic cultures had an equivalent degree of acid tolerance that was greater than that achieved in log-phase cultures grown anaerobically. These results are consistent with the growth-phase regulation of acid tolerance by the stationary-phase sigma factors38. The addition of NaCl (1%) also enhanced acid tolerance of log-phase but not stationary-phase cells of strain 43895. Growth temperature influenced the acid tolerance with progressively greater DpH1.15values obtained at 15, 25, and 37°1C, in both log and stationary phase. Therefore, the influence of temperature on the subsequent survival and acid tolerance of E. coli O157:H7 strain 43895 in ground beef was evaluated. Numbers of strain 43895 decreased c. 1.14 log10cfu g-1in inoculated ground beef stored at 4°1C, whereas numbers remained essentially unchanged during storage at -20°C. While pre-incubation at 15°1C for 4 h prior to storage at 4 or -20°C did not influence survival, the acid tolerance of E. coli O157:H7 survivors was significantly decreased (P<0.10001). These results indicate that the processing temperature can influence acid tolerance in E. coli O157:H7.