Fluctuations of lamellar structure prior to a [Formula Presented] transition in a nonionic surfactant–water system has been investigated by means of small-angle x-ray scattering (SAXS) and differential scanning calorimeter measurements. For large [Formula Presented] [Formula Presented] where T is the temperature and [Formula Presented] the [Formula Presented] transition temperature) in the lamellar phase, the SAXS profiles can be described by a Caillé correlation function for undulating lamellar structure. Approaching the temperature to the [Formula Presented] an excess diffuse scattering grows at the lower Q (Q is the magnitude of scattering vector) side of the first lamellar peak. Highly oriented lamellar samples revealed that the excess diffuse scattering arises from in-plane density fluctuations. We attribute this diffuse scattering to the perforation fluctuation layer (PFL) structure and we show that the PFL is an equilibrium structure. At [Formula Presented] the PFL transformed to the gyroid phase through a transient ordered structure having a rhombohedral symmetry.