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The CAM photosynthetic-pathwayplant Hylocereus undatus) was introduced to the Israeli Negev Desert as a fruit crop with a high water-use efficiency (WUE). Its photosynthetic rate (P-N) was measured continuously in the field with a "photosynthesis monitor" (PM48 made in Israel). The system was closed for a short (2 minutes) sampling period and opened for the rest of the time (28 minutes), when undisturbed gas exchange occurs. Time Domain Reflectometry (TDR) was used to measure evapotranspiration (ET). Quasi stem conductance (QC; kPa(-1)) was determined as the ratio between relative sap flow and VPD, representing the ratio between transpiration and its driving force. Water (equivalent to 2 liters/plant-day) was applied by a drip system every other day. Withholding water decreased PN by about 10% per day. Fluctuating maximal PN reflected the rapid changes in soil water status of the sandy soil. Sinusoidal fluctuations of QC and PN reached their maximum at night. They were zero during daytime when ET was 3.2 mm (60% of potential ET). Due to the very low QC during the daytime, most ET was attributed to evaporation from the soil rather than to transpiration. The low ET and the high of H. undatus resulted in a WUE that was several times higher than the WUE of other fruits crops in the Negev Desert.