A laboratory flow apparatus was used to visualize and measure two-phase gas-liquid flows in natural rough-walled rock fractures. Experiments at carefully controlled flow rate and pressure conditions have been performed using a natural fracture and three transparent fracture replicas. Two-phase flow exhibited persistent instabilities with cyclic pressure and flow rate variations even under conditions of constant applied boundary conditions. Visual observations of changes in pore occupancy showed that the instabilities could be explained as resulting from an interplay between capillary effects and pressure drop due to viscous flow. Measurements of relative permeabilities indicated strong phase interference, with relative permeabilities reduced to very small values at intermediate saturations for both wetting and nonwetting phases. These results run counter to a conventional view of fracture relative permeabilities that assumes that the relative permeability of each phase is equal to its saturation, but the results are consistent with recent models that view fractures as two-dimensional heterogeneous porous media. ¿ American Geophysical Union 1995