Mutants of Lactococcus lactis producing excess carbon dioxide could be isolated on LDHA-20 agar (described by El Attar et al. Journal of Dairy Research67 641–646 2000). The use of these mutants in the manufacture of Roquefort cheese has the potential to improve the formation of openings in this cheese. The aim of this work was to examine the stability of these mutants, their enzymic activities and their metabolism of lactose and citrate during growth in milk. They produced less l-lactate than the parent strain and their lactate dehydrogenase activity was lower. Nevertheless none of the mutants produced no L-lactate at all and the most active gas generators among them generally produced 30–50 mM-L-lactate. Unexpectedly, all the strains produced some D-lactate, some > 10 mM. We found that carbon dioxide production by the mutants could be determined indirectly by assaying acetoin, citrate and 2,3-butanediol by high-performance liquid chromatography. Generally, spontaneous mutants were more stable than those obtained after treating with nitrosoguanidine or u.v. irradiation.