In a greenhouse pot experiment conducted in Turkey during 2001, onion seedlings were transplanted on May 31 at the density of five plants per pot. On this date the soil water content of all pots were at field capacity. The pots were weighed daily until harvest (December 2), and the data were used to determine the daily evapotranspiration and quantity of irrigation. Eight irrigation treatments were applied, designated as I-1 full irrigation (non-deficit treatment), and I-2, I-3 and I-4 no irrigation in the vegetative growth periods, yield formation and ripening, respectively, and I-5, I-6, I-7 and I-8 received 0.0, 0.25, 0.50 and 0.75 times the soil water depletion in the treatment I-1 on the same day. For each treatment, the following parameters were analysed and compared: applied irrigation depth, daily and seasonal evapotranspiration, bulb yield, yield response factor (k(y)), irrigation water use efficiency (IWUE) and water use efficiency (WUE). The findings indicated that onion plants were very sensitive to lack of soil water during the total growing season and the yield formation period, but rather insensitive in the vegetative and ripening periods. High water use and water use efficiencies were observed with increasing levels of irrigation, or no irrigation in the vegetative period. (C) 2004 Elsevier B.V. All rights reserved.