Field experiments were conducted to study the effects of water deficits on potato (Solanum tuberosum L.) evapotranspiration, tuber yield and water-use efficiency. The experiments were carried out under cool season semiarid climatic conditions in Erzurum province located in the east of Turkey in the spring seasons of 2003 and 2004. The experiment included six water deficits: 0% (well-irrigation), 20, 40, 60, 80 and 100% (non-irrigation). Results indicate that deficit irrigation influenced evapotranspiration, tuber yield and water-use efficiency. Potato evapotranspiration decreased as water deficit increased. Seasonal evapotranspiration was 445.2 mm in well-irrigated treatment and 195.2 mm non-irrigated treatment. Potato yield and water-use efficiency were also found decrease as water deficit increased. Potato yield of well-irrigated treatment was 26.43 t ha-1 while 8.28 t ha-1 in non-irrigated treatment. A linear relationship was found between evapotranspiration and tuber yield. Water use efficiency was 59.56 kg/ha/mm for well-irrigated treatment and 42.13 kg/ha/mm for non-irrigated treatment. The seasonal yield response factor (ky) for potato was also found as 1.12.