The osmotic stress technique was used to measure the force between the surfactant bilayers in a nonionic lamellar phase. The bilayer repeat distance, d, was varied in the range 90 < d < 250 Angstrom, corresponding to volume fractions between 0.12 and 0.33. Since the bilayers are uncharged and sufficiently far apart, the only relevant force is the steric undulation force. The obtained relation between force and bilayer separation was compared to Helfrich’s expression for fluctuating membranes, and a good agreement was found in the whole concentration range. Together with the qualitative proof of Helfrich’s expression we also obtained a realistic value of the bending modulus kappa = (2.5 +/- 0.5)k(B)T.