The control of malolactic fermentation (MLF) by Oenococcus oeni is an essential step in winemaking process. Although selected O. œni strains are available for winemakers, the MLF is not always successful, sometimes because of indigenous presence of other strains. In this work, interaction during MLF between five strains of O. œni was studied. Experiments were performed in MRS medium modified to be closer to wine conditions (pH 3.5, 10% ethanol). Interaction in mixed cultures of 10 pairs formed by the five strains were analysed from experiments performed in a Membrane Bioreactor (MBR). Pairs were classified in three different classes among growth interaction: 1) negative reciprocal interaction of both strains (6 pairs), 2) interaction that affect negatively one strain and positively the other (3 pairs), and 3) interaction with positive effect on one strain and no effect on the other (1 pair). Thanks to a mathematical model previously established during pure cultures to link growth and malic acid consumption, effect of mixed cultures on the specific activity of cells to consume malic acid has been equally evaluated. This capacity seemed not to be affected for 4 pairs whereas it seemed activated for 6 pairs.