Following the synthesis of aligned single-wall carbon nanotubes in the channels of AlPO4-5 zeolite single crystals [Appl. Phys. Lett. 73 (1998) 2287], we present the first X-ray diffraction and diffuse scattering results. They can be analysed in terms of a partial filling of the zeolite channels by nanotubes with diameter around 4 Angstrom. The possible selection of only one type of nanotube during the synthesis, due to the constraints imposed by the zeolite host, is discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.