In order to get accurate information about the preincubation time needed for viscous aseptic products, the growth characteristics of three food poisoning organisms Staphylococcus aureus, Clostridium perfringens and Bacillus cereus in pea soup, and one spoilage organism. Lactobacillus plantarum, in tomato soup, were followed during a preincubation period at 30°C for 14 days. The sterile food packs were sealed into polyamidepolyethylene laminate bags containing different headspace gas concentrations (21% oxygen + 80% nitrogen, 5% oxygen + 95% nitrogen and 100% nitrogen). Bacterial growth was followed every second day by counting the number of colony forming units and following the headspace oxygen and carbon dioxide concentrations. The bacterial strains differed in their growth characteristics, especially in their ability to consume oxygen from the headspace, and also in their ability to produce carbon dioxide. The headspace oxygen concentration was also followed by oxygen indicators in order to determine the correlation with the oxygen concentration of the headspace and the growth of bacteria. The bacterial growth followed more closely the concentration of carbon dioxide than the concentration of oxygen. It was concluded that further studies are needed to determine the preincubation period for viscous aseptic products in order to furnish accurate data on the growth characteristics of low numbers of bacteria in aseptic foods at different headspace gas compositions.