This study was undertaken to determine whether Leuconostoc citrovorum plays a role in carbon dioxide production in milk. The ability of L. citrovorum strains to produce gas was studied by two methods. A qualitative method, in which an agar plug was forced up the neck of a volumetric flask, measured gas visually. This method demonstrated that 0.25% yeast extract, in a milk medium at 30 C, stimulated the production of at least 10 ml of gas. Studies using a Gilson Differential Respirometer revealed that L. citrovorum produced 500 to 900 μliters of CO2 in 6.5 hr, whereas 800 to 1,500 μliters of CO2 was produced in nonfat milk which contained 0.33% yeast extract. Cell extracts of Streptococcus cremoris, S. lactis, Lactobacillus lactis, L. casei, and L. helveticus also enhanced gas production of L. citrovorum from 20 to 70%. Autolysates of these bacteria, present during the ripening or ageing of certain cheeses, may stimulate L. citrovorum, a common organism in starter cultures, to produce gas, causing, for example, the slit-open defect of cheddar cheese. Yeast extract caused an increase in acid and gas production per cell but did not cause an increase in growth. Experiments indicated that one metabolic source of carbon dioxide was the decarboxylation of pyruvate produced during catabolism of citric acid. Yeast extract stimulated this reaction by 16%.