Dye-altered beta-1,4 glucan sheets synthesized by the bacterium Acetobacter xylinum in the presence of the fluorescent brightener Tinopal LPW(TM) Were induced to crystallize into cellulose microfibrils by a novel photoisomerization of the dye molecules. Reducing the rate of dye removal led to more uniform microfibril dimensions, narrower ribbon width, and a greater microfibril helical pitch. The induced microfibrils are theorized to represent a composite of parallel glucan chains with dye molecules intercalated and deforming the cellulose lattice. It is speculated that during cellulose synthesis in general, glucan minisheets could associate by a constriction in the extrusion pore of the cellulose synthase complexes.