the properties of cellulose-reinforced nylon-6 block copolymers were studied. composites were prepared by the reaction injection molding (rim) of a nylon system in which one of the reactants contained cellulose fibers. incorporation of cellulose fibers into the liquid reactant of the nylon-rim system resulted in increased viscosity, depending on the amount and the aspect ratio of cellulose fibers. after proper drying, cellulose did not disturb the polymerization of the nylon-rim system. the cellulose fibers increased the stiffness of the nylon-6 block copolymer. the stiffening effect depended on the aspect ratio and surface treatment the tensile strength of cellulose-nylon composites was slightly decreased. elongation at break and impact strength were reduced compared to glass fiber-reinforced nylon. dynamic mechanical measurements showed that the cellulose improved the elastic modulus of the nylon, especially at elevated temperatures. scanning electron microscopy on fractured surfaces showed improved adhesion between fiber and matrix when a surface treated cellulose was used. 16 refs