Kozeny's approximate solution to the problem of fluid flow through porous media is developed and the result is checked by experimental data on air flow through plugs of cotton, wool, rayon, and glass wool fibers. The solution givesWA=k&ggr;02&mgr;&tgr;&sgr;2(1−c)3c2−&dgr;p2&dgr;xfor the isothermal linear flow of a gas.W/Ais the macroscopic flux density; &mgr;, the viscosity; &ggr;0, the density of the gas at unit pressure; &tgr;/&sgr;, the volume of fibers divided by their surface area;c, the fraction of space occupied by fibers; &dgr;p2/&dgr;x, the macroscopic gradient of the square of the pressure; andkis a numerical constant which depends on the shape and orientations of the fluid passages.kwas found experimentally to be 0.18, which is in approximate agreement with the value found for flow through media made up of spherical particles. The dependence of the flow on the factorcwas checked over the approximate range fromc=0.1 toc=more than 0.5. &tgr;/&sgr; andchave values for fineness and density of samples of fibers in fair agreement with independent determinations.