Water requirement for onion (Baia Periforme Precoce Variety) was studied by means of field experiments in which water was supplied by the furrow-irrigation method. The "class A" pan was used to evaluate the water need or this crop. Four treatments were used in which soil water potential was maintained at a minimum of -0.5; -1.0; -6.0 and -15.0 bars. Soil moisture content was controlled gravimetrically. The results obtained led to the following main conclusions: a) yield of onion bulbs increased with soil water potential; b) the following ratios (ER/E0) were determined for the best treatment in which the minimum soil moisture potential was -0.5 bar: 0.64; 0.63 and 0.63 for the second, third stages and the addition of these, respectively.