When flocculating cells of Zymomonas mobilis were put in contact with increasing concentrations of ethanol, flocculation was intensified. The settling velocity increased 5.3-fold when the ethanol concentration rose from 0 to 12% v/v. Additionally, the percentage of free-cells diminished considerably (from 33 to 10%), denoting an increase on the extent of flocculation. The stress provoked by ethanol possibly leads to the activation of genes responsible for aggregation.