The permeability of illite-rich shale from the Wilcox formation has been measured as a function of effective pressure for bedding-parallel flow of 1 M NaCl pore fluid. Permeability k decreases from ~300¿10-21 m2 to 3¿10-21 m2 as effective pressure Pe is increased from 3 to 12 MPa; these values confirm that shales form effective barriers to fluid transport in sedimentary strata over extended geologic times. The variation of k with Pe for Wilcox shale is given by k=k0 <1-(Pe/P1)m>3, where P1=19.3 (¿1.6) MPa and m=0.159 (¿0.007). The value of k0 for Wilcox shale is of the order of 10-17 m2 and may vary among samples by as much as 70%. Effective pressure is given in terms of the external confining pressure Pc and internal pore pressure Pp by Pe=Pc-&khgr;Pp, where &khgr;=0.99 (¿0.06). While our measurements yield &khgr;=~1 for shale with a clay content of ~45%, others have reported &khgr; values for clay-bearing sandstones that rise from ~0.75 to 7.1 with increasing clay content (from 0 to 20%). The trends between &khgr; and clay content revealed by these comparisons imply that the value of &khgr; depends upon the relative distributions of compliant clay minerals and other stiffer minerals. These values of &khgr; also suggest that effective pressures within interbedded sandstones and shales may differ, even at the same equilibrium Pc and Pp conditions. ¿ 2001 American Geophysical Union