alpha-Helix formation of a peptidyl sequence is stabilized by hydrophobic residues recurring at positions which create a longitudinal hydrophobic strip upon folding of the sequence as a helix against a hydrophobic surface. To test that hypothesis, we measured by circular dichroism the helical coiling on lipid vesicles of nine analogs of a prototypic helix peptide PH-1.0, Leu-Tyr-Gln-Glu-Leu-Gln-Lys-Leu-Thr-Gln-Thr-Leu-Lys. In these analogs, Thr was substituted for 1 or 2 Leu residues in the longitudinal hydrophobic strip Leu1...Leu5..Leu8...Leu12 which forms in the alpha-helical configuration. We found that coiling of analogs of Leu-Tyr-Gln-Glu-Leu-Gln-Lys-Leu-Tyr-Gln-Thr-Leu-Lys on lipid vesicles depends upon the strength and structure of its longitudinal hydrophobic strip.