Because permeability increases with the square of the grain size for a given fluid fraction, it is commonly assumed that porous flow of fluids and melts is more effective in coarse-grained domains of deep-seated rocks. This relative behavior is accurate, however, only as a description of two systems operating in isolation. We demonstrate that if coarse- and fine-grained domains are in chemical communication, then equilization of pore-wall curvature across the system results in concentration of fluid or melt in domains of finer grain size. Because of this localization of fluid, the permeability of the finer-grained domains will match or exceed that of domains with coarser grains. Fluid focussing due to differences in grain size has potentially important implications for grain-scale transport of metamorphic fluids-including those released from subducting slabs into the sheared mantle wedge-and also for partial melts in the mantle and deep crust.