To study the effect of ethanol on Oenococcus oeni activity at the single cell level. The active extrusion of the fluorescent probe carboxy fluorescein (cF) was used to assess the metabolic activity of ethanol-stressed O. oeni cells. Subsequent flow cytometric analysis revealed that O. oeni cells extrude the accumulated cF upon energizing with l-malic acid. However, O. oeni cells exposed to 12% (v/v) ethanol for 1 h showed a decreased capacity for active extrusion of cF. Moreover, two subpopulations could be distinguished, one of which being able to extrude cF and the other one remaining cF fluorescent. Growing cells in the presence of 8% (v/v) ethanol resulted in robust cells that maintained the capacity to actively extrude cF after being exposed to 12% (v/v) ethanol, which in turn correlated with the high levels of ATP observed in these ethanol stressed, malolactic fermentation (MLF) performing cells. From our results, it becomes evident that active extrusion of cF can be used to assess malolactic activity in O. oeni. The present study provides information for the development of a rapid method to assess the malolactic activity of individual O. oeni cells performing MLF during wine production. (We recommend that you consult the full text of this article).