Potted 2-year-old 'Hass' avocado trees (Persea americana Mill.) grafted on two clonal rootstocks: Duke 7 (D7) and Toro Canyon (TC) were studied in terms of sap flow rate and anatomical and morphological characteristics of their root systems. Sap flow rate was measured with a Dynagage sap flow system based upon the heat balance technique. Root morphology was assessed by identifying the type of roots (fine roots with only primary growth or thick roots with secondary growth) and the root branching order for each rootstock. Roots were also examined histologically to determine the relative proportion of stele and cortex. Significant differences in the sap flow rate were found among the rootstocks, where Hass, on D7, had a 29%-higher sap flow rate than did TC. Besides, anatomical and morphological differences were found between the different kind of roots in the rootstocks: D7 tended to have finer roots, with a greater proportion of stele than cortex in comparison with TC. The results suggest that the differences in water consumption of 'Hass' on different rootstocks may be associated with root anatomy and its function in water transport.