The rubber tree (Hevea brasiliensis) is an important economic crop, particularly in Thailand. As the plantations extend to new and drier areas, information on physiological parameters such as water use efficiency (WUE) is required to help breeding for adapted genotypes. Carbon isotope composition of bulk leaf (δ¹³C) can be used as an indicator of intrinsic water use efficiency (WUE) of plants. A prerequisite step is to analyse the genetic variability of δ¹³C. We compared the carbon isotope composition of bulk rubber tree leaf (δ¹³C) in rainy and dry seasons in a collection of 49 wild genotypes of rubber in Northern Thailand. There were large δ¹³C variations among the genotypes at all seasons. δ¹³C ranged between −33.3‰ to −30.0‰ in rainy season and −33.1‰ to −29.9‰ in dry season. δ¹³C was rather stable with a good correlation between rainy and dry season. In rainy season, there was a positive significant correlation between δ¹³C, leaf mass per area (LMA) and leaf nitrogen per area (N_A). The average δ¹³C was significantly different between two genetic clusters previously identified in this population. The large genetic variability of δ¹³C in wild rubber population is promising for breeding genotypes using less water.