The beta-chitin microfibrils hom the deep-sea hydrothermal vent worm Riftia pachyptila were studied in both mature and fresh tubes experimentally obtained. The methods used were electron diffraction and electron diffraction contrast images, in conjunction with an electron-energy filter, operated in the zero-loss mode, In both studied samples, microfibrils are organized in successive layers, inside which they are parallel. However, the fresh tube is less densely packed and diffraction data show that these microfibrils may be in a partially hydrated state. These results indicate that a later step occurs in the compaction of the tube material once it has been extruded. Comparison of filtered and unfiltered diffraction patterns shows that zero-loss filtering significantly improves both working conditions and quality of diffraction recordings. (C) 1997 Academic Press.