The metabolism of the disaccharide lactose is of primary importance in those lactic acid bacteria used in dairy fermentations and is reviewed in Fox et al (1990) and Axelsson (1998). Lactose may enter the cell using either a lactose carrier, lactose permease,followed by cleavage to glucose and galactose or via a phosphoenolpyruvate-dependent phosphotransfer-ase (PTS) followed by cleavage to glucose and galactose-6-phosphate. Glucose is metabolised via the glycolytic pathway, galactose via the Leloir pathway and galactose-6-phosphate via the tagatose 6-phosphate pathway. Most L. lactis strains used as starters for dairy fermentations use the lactose PTS the genes for which are plasmid located. Among some thermophilic LAB only the glucose moiety of the sugar is metabolised and galactose is excreted into the medium, although mutants of S. ther mophilus have been described which metabolise galactose via the Leloir pathway (Thomas and Crow,1984; Hutkins and Morris, 1987; Cogan and Hill,1993).