Summary The microflora of traditional yoghurt consists of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. Using two promiscuitive plasmds, conjugational pAMβ1 and non-conjugational pNZ18, both bacterial species were analyzed for their abilities to participate in gene transfer by conjugation, transduction or transformation. While no transfer at all into L. delbrueckii subsp. bulgaricus was observed under any of the conditions tested, S. thermophilus was able to act as recipient or donor in conjugation and transduction. Transduction even occurred in yoghurt. Transformation rates observed for S. thermophilus were low and may be at least partly attributed to the presence of surface associated nuclease activities. Segregational stability of pAMβ1 was shown to depend on environmental factors and on the host bacteria harbouring the plasmid. The data are discussed with respect to safety evaluation and risk analyses of genetically modified starter bacteria, and demands for future research on safety evaluation of novel foods are addressed.