The effects of atmospheres containing 20% O2 and 0 to 80% CO2 or 30% CO2 and 0 to 40% O2 (balance N2) on the growth rate and lag phase of Listeria monocytogenes and Pseudomonas fluorescens in buffered nutrient broth at 7.5°C were investigated. Increasing CO2 concentration lengthened the lag phase and decreased the growth rate of both organisms. CO2 levels above 20% significantly reduced the growth rate of P. fluorescens and CO2 levels of 80% significantly reduced the growth rate of L. monocytogenes . Linear and multiple regression analyses were used to describe L. monocytogenes and P. fluorescens growth curves. The R2 values of all regression analyses were greater than 93%. The regression equations indicate that CO2 inhibits the growth of both P. fluorescens and L. monocytogenes even in the presence of 20% O2, However, the effect of CO2 on P.fluorescens is greater than on L. monocytogenes .