This study used X ray computed tomography (CT) to investigate the variability in the aperture field of natural fractures in two granitic cores; one core, 57 mm in diameter and 146 mm in length, was studied with a medical CT scanner, whereas the other core, 87 mm in diameter and 245 mm long, was characterized by means of an industrial CT scanner. A quantitative methodology for measuring the fracture-aperture thickness from CT images was developed and applied to images of cores to reconstruct the distribution of fracture aperture in the cores, with a spatial resolution of 1.4 mm by 1.4 mm by 5 mm. The CT images of the natural fractures revealed that the aperture thickness (1) is not constant within the fracture plane, (2) varies over several orders of magnitude, and (3) approximately followed a lognormal distribution for one of the cores. The CT method, when applied to monitor the movement of contrast agents injected into the cores, demonstrated fingering of the flow. Higher contrast agent concentrations and earlier contrast agent arrivals correlated with larger aperture regions of the core. The results of this study demonstrate that CT can be used to characterize fractures nondestructively and to detect the movement of contrast agents in granitic cores. ¿ American Geophysical Union 1993