Styrene(S)-isoprene(I) multiblock copolymers of the (SI)(n) type (n = 1, 2, 3, 4) were prepared by a multistep monomer addition technique of anionic polymerization. Molecular weights of SI diblock units are almost the same and the total polystyrene volume fractions are about 0.5. All the film specimens cast from toluene solutions were found to have alternating lamellar structures. The lamellar domain spacing monotonically decreases and approaches an asymptotic value with increasing the number of blocks.