This paper reports on a new laboratory methodology for the determination of the structure of pores, a method for the analysis of the data to obtain the statistics of the pore structure distribution, simulation of porous media with statistics consistent with those in the specimens, numerical simulation of fluid flow in images of porous media, and the determination of permeabilities from the numerical experiments and comparison with the results of laboratory experiments. The computed flow fields show that flow in porous media is restricted to preferential paths depending on the size and connectivity of pores. Whole areas of the pore structure are relatively isolated from the flow because of bottlenecks. The distribution of the isolated regions depends on the angularity of the grains and the porosity of the medium. The more angular the grains are the greater is the possibility for the presence of isolated regions. For grains with the same angularity a decrease in the porosity leads to concentration of flow along preferential flow paths. The permeability tensor coefficients are derived from the flow fields of the two-dimensional images as well as the three-dimensional computer-simulated images of soil microstructure. The numerical values of permeability and permeability anisotropy ratio compare well with laboratory experimental data. ¿ 2000 American Geophysical Union