The structure in the solid state of two helical nylon 3 derivatives, poly(alpha-n-hexyl-beta-L-aspartate) and poly(alpha-n-octyl-beta-L-aspartate), was investigated by molecular mechanics simulation methods. It was found that both polymers adopt the 13/4 helix previously observed for poly(P-L-aspartate)s bearing alkyl side chains of small size. The hexyl derivative could not form stable tridimensional structures. This is in agreement with reported experimental results which showed such polymers to be uncrystallizable. In the case of the octyl derivative, a crystal structure arranged in layers with a rhombic unit cell in the P2(1)2(1)2 space group appeared to be energetically favoured. Side chain trajectories within the interhelical space were found to vary depending on the position of the residue along the repeating unit of the helix. Side chains protruding normal to the sheets are almost extended whereas those located along the sheets are folded. A quantitative estimation of the penetration of the side chains into the intermolecular space has been made on the basis of interatomic distances measurements.