Single polymer composites (SPCs) based on polyamide 6 (PA6) were prepared by in-mold activated anionic ring-opening polymerization (AAROP) of caprolactam in the presence of PA6 textile fibers. The influence of the reinforcing fibers content, their surface treatment, as well as of the temperature of AAROP upon the morphology, crystalline structure, and mechanical properties of the resulting SPCs was followed. The presence of oriented transcrystalline layer (TCL) on the surface of the reinforcing fibers was demonstrated by means of microscopy methods. Its orientation and polymorph structure were determined by synchrotron wide-angle X-ray scattering. Studies on the mechanical behavior in tension of the SPCs showed a well-expressed growth of the stress at break (70-80 %) and deformation at break (up to 150-190 %) in composites with 15-20 wt% of reinforcements. The best mechanical properties were found in SPCs whose reinforcing fibers were solvent-pretreated prior to AAROP in order to remove the original finish. In these samples a stronger adhesion at the fiber/matrix interface was proved by scanning electron microscopy of cryofractured samples. This effect was related to a thinner TCL in which the alpha-to-gamma polymorph transition is impeded.