The structure of poly(L-lysine) containing 44% azobenzene units in the side chain was studied by X-ray diffraction between room temperature and 150 degrees C. The polymer exhibits a mesomorphic structure of the smectic Al type. In this structure, stable at least until 150 degrees C, each smectic layer of thickness d results from the superposition of two layers: one of thickness d(A) contains the free lysine side chains, the other of thickness d(B) contains the azobenzene modified lysine side chains and the polypeptide main chains, that in their planes are arranged as in the ''antiparallel'' beta-structure classical for polypeptides.