Conjugation between lactose-fermenting (Lac+)Streptococcus lactis C2 and Lac− Leuconostoc cremoris CAF7 was performed. The frequency of Lac+ transfer was 1.5 · 10−2 per donor cell. Lac+ Leuconostoc transconjugants could ferment lactose significantly faster than wild-type cells. When grown in litmus milk fortified with 0.2% yeast extract, Lac+ transconjugants reached pH 4.68 within 24 h at 30°C and produced diacetyl. The identity of the transconjugants asLeuconostoc derivatives was confirmed by their resistance to phage c2 and to vancomycin (>500 μg/ml), and by growth on selective medium containing azide. Plasmid profiles of 10 transconjugants showed two unique patterns. A novel enlarged plasmid was found. Southern blot hybridization revealed some homology with the 30 Md Lac+ plasmid of donor, recipient and the transconjugants, as well as with some of the remaining plasmids of the donor.