Chitosan was reacted with 1-bromooctadecane to yield N-octadecylchitosan consisting of 70 mol % of 2-(octadecylamino)-2-deoxy-D-glucopyranose, 17 mol % of 2-amino-2-deoxy-D-glucopyranose (GlcN), and 13 mol % of 2-acetamido-2-deoxy-D-glucopyranose. N-Octadecylchitosan shows a gel to liquid-crystalline phase transition in a differential scanning calorimetry (DSC, T-c = 46 degrees C) and forms bilayer membranes with 50 Angstrom thickness (electron microscopy) in acidic water by sonication. Polymeric bilayer membranes of N-octadecylchitosan were covalently immobilized to carboxylated porous supports by amidation between primary amino groups of GlcN moieties and carboxyl groups of the supports. Electron microscopy and DSC of the supports suggested the retention of membrane structure upon the immobilization. The resulting porous supports bearing polymeric bilayer membranes are stable to 0.5 N NaOH or 0.1 N HCL. The amount of the released organic carbon from them during a washing with water for 24 h was less than 0.04 % of the organic carbon of the immobilized N-octadecylchitosan.