The elastic moduli E-l of the crystalline regions of alpha-chitin and chitosan in the direction parallel to the chain axis were measured by X-ray diffraction. The E-l values were 41 GPa for alpha-chitin, and 65 GPa for chitosan, respectively, at 20 degrees C. The contracted skeletons of alpha-chitin and chitosan are the key factor for the low E-l values compared with that (138 GPa) of cellulose I. The E-l value of a-chitin was constant at 41 GPa both at - 190 degrees C and 150 degrees C, which indicates that the molecular chain of a-chitin is stable against heat within the temperature and stress range studied.