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The objective of this research was to evaluate the diversity of xylem sap flux density in four immature hybrid rubber clones, including the main mother clone RRIM600, planted in northeast Thailand. These hybrid clones, bred from the original RRIM600 (female) and RRII105 (male) clones, differed in trunk diameters. The age of the sample trees was 4 years. Measurements were taken from April to October 2016, within the full-leaves stage. The experimental design consisted of a randomized block design of hybrids and blocks for performing measurements at four environmental conditions: well-watered and sunny day (VPD 2.2 kPa), well-watered and extremely sunny day (VPD 3.2 kPa), well-watered and cloudy day (VPD 0.9 kPa) and dry soil and extremely sunny day (VPD 4.2 kPa). Sap flow density was recorded through the transient thermal dissipation method (10 min heating and 20 min cooling in a half hour period; TTD10) with a Granier's type probe. The results show that the maximum and daily sap flux densities were reduced by 40-45% on the cloudy day and soil drought conditions, compared to the well-watered and sunny day condition, while daily sap flux density in the extremely sunny day condition slightly increased. Differences in trunk size were insignificant in relation to the varying sap flux densities.