Effects of different chemical compounds on ciliates grazing outdoor mass cultures of Dunaliella salina were studied. These taxonomically undetermined ciliates were capable of clarifying dense algal cultures within 2 days and tolerated a wide salinity range (up to 215 ppt). Treatments of micro-algal cultures with either formaldehyde, ammonia, hydrogen peroxide or metronidazole were inappropriate since lethal doses for the ciliate also eliminated algal cells. However, the use of quinine sulphate rapidly destroyed ciliates while keeping algal cells alive. A 24-h LC100 at 12-14 mg l - 1 quinine sulphate was found for the ciliate, whereas a 72-h EC50% at 14.5+/-0.6 mg l - 1 was found for Dunaliella. The application of a dose of 10 mg l - 1 quinine to contaminated outdoor algal mass cultures completely eliminated ciliates within a short time, and allowed the algal population to start recovering.