Growth responses of young `Hamlin' orange [Citrus sinensis (L.) Osbeck] on sour orange (C. aurantium L.) trees to microsprinkler irrigation were studied under field conditions from 1985 to 1987 to determine the most-efficient irrigation rates and duration. Trees were irrigated when available soil water depletion (SWD) reached 20% (high frequency), 45% (moderate frequency), and 65% (low frequency). Trees at the moderate and low levels received 49% and 13%, respectively, as much irrigation water as the high treatment. Canopy volume, trunk cross-sectional area, dry weight, shoot length, leaf area, total root dry weight and volume, and new root dry weight were similar for the high and moderate levels in 2 of 3 years, but were significantly reduced at the low level. Summer and fall growth flushes were delayed or did not occur at the moderate and low levels. More than 90% of root dry weight was within 80 cm of the trunk at the end of the first growing season.