We have investigated the effect of the insertion of a triblock peptide (hydrophobic-hydrophilic hydrophobic) in a nonionic lamellar phase composed of C12E4, decane, and water, stabilized by bilayer thermal fluctuations. Circular dichroism shows the peptide to be unordered in water, whereas its hydrophilic part is rigid and organized in an a-helix in the presence of surfactant bilayers. Surface tension measurements prove that the peptide is located at the hydrophobic-hydrophilic interface. Together with spectrofluorometry, these experiments suggest that the peptide lies on the bilayer surface. The Caille parameter, 1, of the lamellar phase, obtained by SAXS experiments, decreases with peptide concentration. This decrease has been interpreted as an increase of the bilayer effective thickness induced by the peptide and is well fitted by a recent model. The bilayer bending rigidity K increases linearly with peptide concentration, up to two times the rigidity of a bare bilayer with mole ratio of peptide to surfactant as low as 5.2 x 10(-4). The smectic compressibility modulus, B, decreases, implying that the peptide presence softens interactions between bilayers.