Two methods for measuring aperture distributions within rough-walled fractures are compared; magnetic resonance imaging (MRI) and spectrophotometric analysis (SA) of epoxy replicas. Comparisons with observations of flow through the fracture for Reynolds numbers less than one indicate that the resolution and accuracy of SA is sufficient to predict the effective transmissivity of the fracture using the locally applied cubic law. Observed effective transmissivities for higher Reynolds number flows are lower than predicted using the local cubic law. MRI apertures are generally consistent with SA, except for those less than 100 microns which were not detectable. The lower resolution of the MRI data results in a poor estimate of the effective transmissivity, indicating that despite their low transmissivities, the smallest apertures (<100 microns in this fracture) must still be accurately measured to predict flow through the fracture. ¿ 2000 American Geophysical Union