An interesting inclusion compound (IC) between guest poly(epsilon-caprolactam) (PepsilonCL) and host urea was successfully obtained, for the first time, by co-crystallization from their common solution. X-ray diffraction, infrared spectroscopy and differential scanning calorimetry have been utilized for a detailed structural investigation of PepsilonCL-urea IC (U IC) crystals. The results were compared with those obtained for well-known structures of the hexagonal polyethylene-U IC, the trigonal polyethylene oxide-U IC and the 'large tetragonal' poly(propylene)-U IC. The structure of PepsilonCL-U1C reconfirms that the urea host molecules may crystallize, even in the presence of a rather slim polymer guest, into an IC with a lattice channel diameter of more than 5.25 Angstrom.