The molecular and crystal structure cf curdlun triucetute, acetylated ( I + 3) p-D-glucan, was unul-vzed by means of un x-ray diflraction technique with the help of the linked-atom leastsquares method. Unit cell dimensions are a = b = 1 l . O O ( l ) , clfiber axis) = 22.91 ( 9 ) k, and y = 120". The space groiip is P6,. The unit cell contains six chemical repeating units related by h/l-helicul symmetry, which is essentiully the same as the backbone conformation ofone of the mod$cutions ('ftwm I ) of curdlan. During the refinement calculation, the terminal methyl in every acetyl moiety was elastical1.v restrained to the trans conformation commonly observed in related oligosaccharide striictures. The difference Fourier map, the observed and calculated densities, and the thermogravimetric measurement indicated one water molecule per glucose residue. The water oxygen is linked to two carbonyl oxygens in adjacent molecules by hydrogen bonds. The conftwmation ofthe primary acetyl moiety is a (skew, -gauche, trans). So far, no skew' conformation was observed,for the primary acetyl and hydroxyl moieties except in a, ppanose. In both cases, the unir.siiu1 eclipsed orientation qf the primary group is attributed to the hydrogen bond and this conformation is quite diffcrent,from that ofpachyman triacetate.