Zygosaccharomyces bailii is a yeast that spoils foods by CO2 accumulation. Gas is mainly produced by alcoholic fermentation. To study the extent and regulation of fermentative CO2 production in this yeast, we have evaluated the relevance of the effect of oxygen (Pasteur effect) and sugar (Crabtree effect) concentrations. In controlled-batch cultures on glucose and fructose, the main difference observed was that catabolism on fructose was not affected by oxygen concentration while on glucose, ethanol production increased in the absence of oxygen. This behavior can be explained by a higher capacity to transport fructose and the Crabtree effect. Manometric experiments with resting cells confirmed the limited respiratory capacity of Z. bailii and that, under these nongrowing conditions, cells retained the ability of producing an important amount of gas. The Pasteur effect was not important in Z. bailii since the glucose kinetic parameters in continuous cultures Ir ere trot significantly affected by the oxygen concentration Continuous culture experiments also that the amount of gas produced depended on the oxygen concentration and the sugar consumption rate. For low sugar concentrations which imply low consumption rates, oxygen concentration was an important controlling factor; however, at high sugar consumption rates, gas production became independent of oxygen concentration, or in other words, regulation was dominated by the Crabtree effect.