Dispersion of dissolved contaminants in fractured rocks is strongly dependent on the variability of fracture aperture. Large fracture aperture regions result in significant channeling of the fluid flow, accelerating the movement of solutes in a particular direction, which may differ locally with respect to the main flow direction from high to low hydraulic potential. This may result in very early breakthrough of the contaminants, significantly earlier than in the conventional parallel plate simplification of fluid flow in fractured systems. In addition, flow and transport in the low aperture regions can be orders of magnitude smaller, which has major implications for remediating contaminated fractured media. In this study we observed the transport of a nonsorbing solute in a naturally fractured rock which was characterized at very high resolution using computed tomography scanning to determine the fracture aperture distribution. We compared the observed breakthrough curve of the solute at various flow rates with the predicted breakthrough curve using the fracture's geostatistics and small perturbation analysis, as well as direct numerical simulation of flow through the fracture using the measured fracture aperture distribution. The experimentally determined fracture transmissivity and dispersivity are within a factor of 2 of those predicted using either the small perturbation approach (using the fracture geostatistics) or explicit numerical modeling of breakthrough, which differ by around 20%, indicating that the geostatistics of the fracture aperture can be used to predict the approximate shape of the breakthrough curve for a fracture. ¿ 1999 American Geophysical Union