The effect of varying the molecular weight of butyrylchitin membranes cast from methylene chloride on their properties has been investigated. Characterization analyses include Xray, scanning calorimetry and tensile stress-strain. The crystal structure of the butyrylchitin was shown to consist of fringed micelles, irrespective of the molecular weights examined, and of lower crystal volume fraction than usually found for chitin membranes, 0.3 versus 0.5, respectively. The membranes exhibit greater ductility and less swelling in physiological salt solution with increasing molecular weight.