Water use efficiency (WUE) is essential to agricultural production in arid and semiarid regions. Plant productivity in these regions is directly related to the availability of water. Pinus eldarica Medw. has gained rapid popularity as a short rotation Christmas tree or ornamental crop in Southern New Mexico. The tree also has potential for woody biomass production. Because the tree is a recent introduction to the area, effective irrigation guidelines for its production were unavailable as were guidelines for irrigated pine tree plantations in general. The objectives of this study were to determine crop evapotranspiration (ETc) (mm-day-l 6.25 m-2) and water use efficiency (WUE) (kg DM-m-3 H2O) for above ground biomass of P. eldarica under three flood irrigation regimes. Irrigation regimes equivalent to 173, 298, and 457 mm in addition to 159 mm precipitation during the 1983 growing season were applied. The resultant Etc was 527.9, 558.7 and 604.4 mm in the respective treatments. Much of the water was lost as evaporation from the soil surface due to the widely spaced test plots. Trees in the lowest irrigation regime relied heavily on stored soil water to maintain growth rates similar to other regimes. Height growth increased 0.80, 1.04 and 1.05 m, while biomass increased 3185.6, 3524.8, and 4323.2 kg DM-ha-1 in the 60, 80 and 100 percent irrigation regimes. Water use efficiency ranged from 0.32 to 5.18 kg DM-m-3 H20 consumed as ETc.