Ternary fibrillar reinforced blends are obtained by melt-blending of poly(ethylene terephthalate) (PM), polyamide 6 (PA-6) and polyamide 66 (PA-66) (20/60/20 by weight) in the presence of a catalyst, followed by cold drawing of the extruded bristles to a draw ratio of about 3.4 and additional annealing of the drawn blend at 220 or 240 degreesC for 4 or 8 h. The blend samples are studied by DSC, X-ray diffraction, SEM, and static and dynamic mechanical testing (DMA). SEM and DMA show that PA-6 and PA-66 form a homogeneous, continuous matrix in which PET regions are dispersed. X-ray and DSC measurements of the drawn and annealed at 220 degreesC samples suggest mixed crystallization (solid solubility) of PA-6 and PA-66, and cooperative crystallization of PET with the two polyamides. After annealing at 240 degreesC (above the melting point of PA-6 and below that of PET), the polyamide matrix becomes partially disoriented, while the oriented, fibrillar PET is preserved and plays the role of a reinforcing element. The DSC results for the same samples suggest in situ generation of an additional amount of copolymer. This additional copolymerization, together with that generated during blend mixing in the extruder, improves the compatibility of the blend components (mostly at the PET-polyamide interface) and alters the chemical composition of the blend.