Enzymic digestion, acidic hydrolysis, deuterium exchange, infrared absorption spectra, and differential thermal analysis of chitins show that not only do the structures of <x- and ~-chitins differ but that differences occur within the <x-chitins and within the ~-chitins. The crystalline structure within an <x-chitin varies and about 30% of the chitin is in a highly crystalline form. ~-Chitins prepared from cuttlefish shell and squid pen contain significant amounts of free amino groups, on average one residue in five and two residues in five respectively being unacetylated. The free amino groups are in the less highly organized parts of the structure. The structures of the chitins and their associations with proteins are discussed.