The infrared thermometer used to measure crop water stress index (CWSI) is a reliable tool for irrigation scheduling, which, combined with efficient irrigation systems can maximize crop productivity. A study was conducted to determine the crop water stress index in husk tomato, or tomatillo, (Physalis ixocarpa Brot.) under a drip irrigation system, its relationship with irrigation depth and plastic mulch in scheduling irrigation and predicting fruit yield. The experiment design was completely randomized with three replicates Treatments consisted of five irrigation depths (replacement of 40, 60, 80, 100 and 120 % of the reference evapotranspiration estimated by the Penman-Monteith method). CWSI was estimated using infrared radiation gun measurements of canopy temperature, air temperature, and relative humidity, and water vapor pressure deficit was calculated. The equation which defines the lower limit expresses the relationship between vapor pressure deficit (VPD) and temperature difference (crop and air (Tc-Ta)). When the crop transpires, the relationship is: Tc -Ta = 1.21 - 131 DPV (r2 = 0.68, P <0.01, n = 42), and the upper limit (stressed) was 2.8 °C, when transpiration stops. Fruit yield showed a positive linear correlation with average CWSI values: Y = 52.53-69.7CWSI, (r2 = 0.65, P<0.01 and n=30). Prediction models of CWSI and means of the effect of irrigation water and plastic mulch were fit with r2 = 0.87 to 0.96, P<0.01 and n=30. The CWSI increases linearly when the soil water potential decreases.