Abstract: The response of onion (Allium cepa) to different irrigation schedules was studied in Trakya, Turkey, during 1997 and 1998. Onion crop was subjected to four irrigation treatments according to available soil water depletion fractions (0.30, 0.50, 0.70, and no irrigation). Irrigation thresholds (amount of soil water at 0.40 m depth) were used as criteria to initiate drip irrigations. For each differential water treatment, the parameters of bulb morphology (diameter and height), solids soluble in bulbs, bulb weight, and total yield were analysed. Yield and yield components except solids soluble in bulbs were affected by irrigation and soil water depletion fractions. The highest yield was obtained from the plots to which irrigation water was applied at a soil water fraction level of 0.30. The maintenance of soil moisture depletion level at 0.30 required 339.4 mm (in 14 applications) and 227.2 mm (in 13 applications) of irrigation water in 1997 and 1998, respectively. The seasonal evapotranspiration of onion was 42