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.