The objective of this work was to determine permeability behavior of coal fracture systems as a function of applied stress. This seemed important in connection with the problem of flow of methane gas through coal seams into mine workings. Efforts to counteract the possible consequences of this problem may be aided by knowledge of the flow behavior of typical coals measured under simulated subsurface stress conditions. Three bituminous coals having large differences in hardness and degree of fracturing were selected for the study. Special t techniques were developed for the preparation of two-inch diameter cylindrical test specimens with minimum disturbance of the fracture system. The general flow properties were evaluated by flowing nitrogen axially through the test specimens under various conditions of applied axial and radial stress. A few radial flow tests were run to evaluate the effect of flow direction. Final tests were made by flowing methane gas axially through cylindrical test specimens under a specific sequence of stress conditions. Sonic velocities were measured concurrently with permeability tests. Principal results of this investigation may be summarized as follows. Permeabilities of all three coals were found to vary over a wide range—from about 0·1 to nearly 100 millidarcys at the lowest stress level. Permeabilities were strongly stress dependent decreasing by more than two orders of magnitude in the stress range of 250–2000 psi. Difference in the type of stress application, hydrostatic or triaxial, appeared to have little effect on permeability reduction. The mean effective stress level was found to be the controlling factor in permeability reduction. Permeability of fractured coal was very stress-history dependent, decreasing in magnitude with each loading cycle, except in cases where the applied stress caused further fracturing. Permeability of fractured coal to the flow of methane gas was found to 20–40 per cent lower than the permeability to nitrogen. This amount of reduction can hardly be explained on the basis of molecular diameter or sorption of the gas on fracture surfaces. Further investigation of this behavior is needed. Sonic velocity showed some general trends with change in stress, related most likely to fracture closure. Neither the magnitude of permeability nor the change of permeability with stress could be predicted from sonic velocity behavior. However, an equation was derived from experimental data which gives a good representation of the effects of stress on permeability, providing permeability at zero stress is known.