At the Scoresby Horticultural Research Station in the 1973-74 season an empirical formula was developed to relate the water needs of young peach trees to tree size as measured by butt area and evaporation demand as measured with a Class A pan evaporimeter. The formula was expressed as litres per cm2 of butt area per cm of evaporation. Nine rates of irrigation were tested. These rose by 0.5 litre increments from 0.5 litres to 4.5 litres. Trees were selected over a wide size range. Irrigation quantities were adjusted fortnightly to butt area. The smaller the initial size of the tree at planting the lower the rate of irrigation required for maximum growth, but with the tree sizes covered in the trial (a range in initial butt area of 0.54 cm2 to 3.68 cm2) 95 per cent of the maximum growth response occurred between irrigation rates of 2.5 and 3.5 litres per cm2 of butt area per cm of evaporation. At the higher irrigation rates the bigger the tree at planting the greater the response to irrigation.