An extrudable mixture is provided for producing a highly porous substrate using an extrusion process. More particularly, the present invention enables fibers, such as organic, inorganic, glass, ceramic or metal fibers, to be mixed into a mass that when extruded and cured, forms a highly porous substrate. Depending on the particular mixture, the present invention enables substrate porosities of about 60 % to about 90 %, and enables process advantages at other porosities, as well. The extrudable mixture may use a wide variety of fibers and additives, and is adaptable to a wide variety of operating environments and applications. Fibers, which have an aspect ratio greater than 1, are selected according to substrate requirements, and are mixed with binders, pore-formers, extrusion aids, and fluid to form a homogeneous extrudable mass.; The homogeneous mass is extruded into a green substrate. The more volatile material is preferentially removed from the green substrate, which allows the fibers to interconnect and contact. As the curing process continues, fiber to fiber bonds are formed to produce a structure having a substantially open pore network. The resulting porous substrate is useful in many applications, for example, as a substrate for a filter or catalyst host, or catalytic converter.