The effect of fiber orientation on the toughening of polymers by short glass fibers generally below their critical length was investigated using specimens with either well-aligned or randomly oriented fibers. The fibers were aligned by an electric field in a photopolymerizable monomer, which was polymerized while the field was still being applied. These materials were fractured with the aligned fibers in three orientations with respect to the crack plane and propagation direction. Specimens with fibers aligned normal to the fracture plane were the most tough, those with randomly oriented fibers were less tough, and those with fibers aligned within the fracture plane were the least tough. The fracture behaviors compared favorably with predictions based on observed processes accounting for fiber orientation. The processes considered were fiber pull-out (including snubbing), fiber breakage, fiber-matrix debonding, and localized matrix-yielding adjacent to fibers bridging the fracture plane. Fibers not quite perpendicular to the fracture plane provided the greatest toughening; these fibers pulled out completely and gave a significant contribution from snubbing. Fibers at higher angles provided less toughening, involving nearly equal contributions from pull-out, breakage, and debonding. Fibers within the fracture plane provided the least toughening, involving debonding alone. (C) 2003 Wiley Periodicals, Inc.