A novel family of graft-copolyamides was prepared in which several chains of high molecular weight polycaproamide (nylon 6) are grafted onto the backbone of each stiff aromatic polyamide chain. The unique feature of the present graft-copolyamides is that the side chains are connected through amide groups directly to the aromatic rings in the stiff chain, and not to the amide groups in the backbone of the stiff chain. Because they are miscible with linear nylon 6, blends of the graft-copolymers and nylon 6 are easily prepared. Electron microscopy, x-ray diffraction scans, DSC runs, and solid-state NMR relaxation and spin diffusion experiments all indicate that the aromatic stiff polyamide chains are molecularly dispersed in the graft-copolyamides and their blends with nylon 6. In an ungrafted blend of the stiff aromatic and flexible aliphatic nylon 6, the stiff chains tend to agglomerate into small hydrogen-bonded sheaf-like aggregates the size of which increases upon annealing above the melting point of the flexible polyamide. The NMR results point to a substantial flexibility of the stiff chains in the graft-copolyamides, helping to explain the levels of reinforcement reflected in the tensile properties of these materials. In the ungrafted blends, the aromatic-chain stiffness is greatly enhanced when these chains are phase separated in the minute sheaf-like particles. The tensile properties of the graft-copolymers and their mixtures with pure nylon 6 are far superior to those of the ungrafted blends. The reinforcement of the flexible matrix by the stiff polyamide chains is explained in terms of the Halpin-Tsai formulation.