Ozone (O3) and soil water deficit are two environmental stresses that significantly affect the growth and yield of alfalfa. However, little is known of the responses of field-grown alfalfa to O3, and the effects of the interaction between O3 and water stress on canopy temperature and water relations of alfalfa have not been previously reported. Water use, rate of soil water depletion, or depth of effective rooting zone were not affected by O3, whereas water stress significantly reduced all three. Water use efficiency was significantly reduced by O3. The effects of O3 on WUE were attributed to premature senescence and abscission of older alfalfa leaves.