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The relationship between the timing and amount of applied irrigation water and grain yield has not been well defined for durum wheat (Triticum turgidum L. var. durum) grown under irrigation in arid regions. With rising water costs and decreasing water supplies, grain growers will have to optimize yield for given levels of water inputs if they are to remain profitable. Small-plot field studies were conducted in 1986 and 1987 at Marana, AZ, to (i) test the feasibility of using the crop water stress index (CWSI) to schedule wheat irrigations and (ii) determine relationships among CWSI, grain production, and water applied. Highest grain production was attained when irrigations were scheduled at CWSI values averaging 0.37 units in 1986 and 0.30 units in 1987. Scheduling irrigations with lower CWSI values did not increase grain production, but required more water. Delaying irrigation by scheduling irrigations at CWSI values exceeding the optimum values reduced grain production.