The effects of irrigation (supplied throughout the crop growth period or during critical periods only) and plant density (250 000 or 400 000 plants/ha) on the grain yield of soyabean were studied from January to May 2005 in Santa Maria, Rio Grande do Sul, Brazil. The grain yield did not significantly vary between the plants irrigated throughout the growth period and plants irrigated during critical periods only. The grain yield of non-irrigated plants increased by 21% when the population was increased to 400 000 plants/ha.