The use of soy protein presently is limited mostly to animal feeds, but the markets for soy protein could be expanded by industrial applications. In these studies, soy fibers were produced from soy protein isolate using both wet spinning and extrusion methods. The wet spinning process consisted of forcing an aged alkaline protein solution through a spinnerette having 368-[mu] openings into an acid coagulating bath. The extrusion process employed a twin screw extruder to force a protein-water mixture with "dough-like" consistency through a die also with 368-[mu] openings. The physical properties of the fibers were measured at 11% and 65% relative humidity and wet conditions using an Instron Universal Testing Machine. The fibers produced by both methods initially were brittle and very weak. The addition of glycerol reduced brittleness in extruded fibers and the inorganic ions zinc and calcium decreased the brittleness of wet-spun fibers. Modification of soy proteins by acylation with acetic anhydride or esterification with low molecular weight alcohols prior to extrusion to block the polar groups of soy proteins decreased the tenacity of the fibers. The tenacity of soy fibers was significantly improved by finishing treatments after extrusion using reagents such as acetic anhydride, glyoxal, glutaraldehyde or a combination of glutaraldehyde and acetic anhydride. The fibers finished with acetic anhydride or glutaraldehyde after extrusion became less polar by blocking the polar groups or cross-linking protein molecules. The conditions of finishing fibers and stretching fibers were studied to improve fiber properties. Scanning electron micrographs of the fibers revealed that strong fibers were smooth and continuous while weak fibers often contained cracks and voids.