AbstractExperiments on sphere‐filled aligned fiber reinforced polyester resins show that the Young's and flexural moduli of the composites are increased by the presence of the spheres. The increase is particularly marked for composites made with flexible resins, and has potential for improving the properties of aligned fiber laminates. The moduli in the absence of fibers do not fit well with existing theory, and agreement with finite element analysis is only observed in the case of the rigid polyester. When fibers are present synergistic effects are observe