Vegetable soybeans [Glycine max L (Merr.)] are very sensitive crops to environmental conditions during their growth stages, especially in term of water scarcity. Water scarcity is one of the major environmental factors influencing sustainable agricultural production in arid and semi-arid regions. Careful management irrigation strategies need to save irrigation water with marginal yield reduction. The objective of this research was to investigate the effects of the water deficit on yield and yield components of soybean in semi-arid conditions. This research was carried out at the Agricultural Experimental Field of the Harran University (Sanliurfa, Turkey) on clay soil during the growth periods of 2006 and 2007. The irrigation treatments were 33% (I-33), 67% (I-67), 100% (I-100) and 133% (I-133) ratios of total irrigation water applied (IW)/cumulative pan evaporation (CPE) with four day irrigation interval. The average amount of irrigation water applied to treatments (I-133, I-100, I-67 and I-33) was 1058, 795, 533 and 263 mm and 1094, 823, 551 and 272 mm for Toyokomachi and Toyohomare cultivars, respectively. The maximum green pod yields were 20.6 and 29.1 t ha(-1) with 997 and 922 mm water consumption for Toyohomare and Toyokomachi, respectively in I-133 treatments. Yield response factor (k(y)) values of I-100, I-67 and I-33 treatments were determined as 2.17, 0.92 and 0.59 for Toyohomare and 3.50, 0.61 and 0.61 for Toyokomachi, respectively. The results of the study implied that at least equal (I-100) or excess of the evaporated water amount is required to produce high yield in soybean. Differences of yield between cultivars in response to irrigation levels make it necessary to select less sensitive cultivars to water stress especially in semi-arid and arid areas. Varietal characteristics must be considered for successful growing of soybean.