Secondary salinization of drylands is of world-wide environmental concern. Cultivation of olive is highly encouraged in Mediterranean climates, due in large part to its salt tolerance, which nevertheless varies among cultivars. Most previous studies have addressed above-ground parameters or have been conducted on seedlings, while knowledge of the plasticity of mature root systems under salinity remains scarce. Two olive (Olea europaea L.) cultivars, 'Barnea' and 'Proline', known as salt-tolerant and salt-sensitive, respectively, were examined for the influence of salt stress on the root system. Two levels of saline water were used for irrigating mature trees (EC 1.2 and 4.2 dS m -1). Fine-root biomass and necromass, axial root conductivity and root sapflow density were analysed by means of soil cores, conductivity measurements and miniature sap-flow gauges, respectively. 'Barnea' trees were found to sustain a higher fine-root biomass under salt stress than 'Proline' trees. The mean specific conductivity of 'Proline' fine roots was higher than in 'Barnea', and 'Proline' coarse roots' specific conductivity increased due to salinity. While there were no significant differences between cultivars in root sap-flow density under freshwater irrigation and sap-flow dropped under salt stress in both cultivars, 'Barnea' roots' sap-flow density remained higher than that of 'Proline' under salinity. The olive cultivars showed striking differences in their below-ground reaction to moderate salinity. Salt-tolerant 'Barnea' and salt-sensitive 'Proline' trees differed in both biomass and functionality of their root system. As a consequence, 'Barnea' was able to sustain higher wateruptake rates under salinity, possibly due to the higher remaining root biomass which is key to surviving osmotically induced water scarcity. Future field studies on woody species are urgently needed, addressing parameters that limit water supply in saline soils.