It has long been recognized that various dairying-relevant properties of mesophilic lactic streptococci are unstable traits. During the past ten years it has become increasingly clear that these instabilities are due to loss of plasmids during growth of the bacteria. The plasmid-linkage of these properties has accelerated the identification and delineation of their genetic determinants and recently genes important for dairying have been cloned and expressed in a number of bacterial species, including plasmid-free lactic streptococci. The prospects for improving dairy starter cultures by enhancing and stabilizing the expression of dairying-relevant plasmid-borne genes look promising.