The structures of microbial cellulose-fluorescent brightener complexes, produced from the Acetobacter culture in the presence of a fluorescent brightener, and the cellulose samples regenerated from them have been examined by cross-polarization/magic-angle spinning (c.p./m.a.s.) C-13 n.m.r. spectroscopy. C4 and C6 resonance lines for the cellulose components of the complexes appear at about 84.0 and 63.0 ppm, respectively, with the disappearance of their downfield crystalline components. Since the chemical shifts of the lines are in good accord with those of the non-crystalline component of native cellulose, it is concluded that the cellulose component of the complex is in the non-crystalline state. C-13 spin-lattice relaxation time (T-1C) measurements have also confirmed that the T-1C values of the dried complexes are of the same order as those for the non-crystalline component of native cellulose. In contrast, the T-1C values of the non-dried complex are much shorter than those of the dried complexes, indicating a much enhanced molecular mobility in the non-dried complex. On the other hand, when the non-dried complex is subjected to dye extraction, C4 and C6 resonance lines assignable to the crystalline component can be observed in the c.p./m.a.s. spectrum. Moreover, selective measurements of the spectrum of the crystalline component have revealed that cellulose I beta is preferentially grown from the non-dried complex by dye extraction. In the case of the dried complexes, only very small amounts of cellulose I crystals are regenerated, possibly as a result of the formation of tight hydrogen bonds in the complex.