The molecular directionality of chitin biosynthesis was investigated by transmission electron microscopy (TEM) using electron crystallography methods applied to reducing-end-labelled beta-chitin microcrystals from vestimentiferan Lamellibrachia satsuma tubes and nascent beta-chitin microfibrils from the diatom Thalassiosira weissflogii. The data allowed confirmation that the microfibrils were extruded with their reducing end away from the biosynthetic loci, an orientation consistent only with elongation through polymerization at the non-reducing end of the growing chains. Such a chain-extension mechanism, which has also been demonstrated for cellulose and hyaluronan, appears to be general for glycosyltransferases that belong to the GT2 (glycosyl transferase 2) family. The data also allowed confirmation that in beta-chitin the chains are crystallized in a 'parallel-up' mode, in contrast with hypotheses proposed in previous reports.