Six new sequential hydrophobic polypeptides consisting of leucyl and isoleucyl residues were synthesized by polycondensation of peptide active esters. Solid samples of these polypeptides were obtained by fast reprecipitation from a solution in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) or HFIP/dichloroacetic acid with diethyl ether. The conformation of the solid sample was determined by IR spectroscopy as follows : the beta-structure for (Leu-Ile-Ile-Leu)(n), a mixture of the beta-structure and alpha-helix for (Leu-Ile-Leu)(n), and the alpha-helix for (Ala-Ile-Ile-Leu)(n), (D-Leu-Ile-Ile-Leu)(n), (Leu-Ile-Ile-Gly)(n), and (Leu-Ile-Ile-Aib)(n). The factor governing the specification of the beta-structure/alpha-helix was deduced to be the consecutiveness of the tidily aligned hydrophobic side chains of the amino acid residues to afford sterically well-regulated intermolecular aggregation of the polypeptides prior to the conformational preference of the individual amino acid residue.