Studies of radio waves (2.22–2.66 MHz) partially reflected from the lower ionosphere (50–90 km) have been used to calculate the magnitudes of the Fresnel irregularities required to give the observed signal strengths. These irregularities require 1–18% perturbations in electron density and show a distinctive seasonal variation, having a minimum in summer and a maximum in winter. Recently published electron-density profiles obtained from rocket probes have revealed fine-scale structure comparable in magnitude and vertical dimension to these calculated perturbations. The reflection coefficients and the magnitudes of the equivalent Fresnel irregularities are calculated for individual irregularities and are shown to be only 3 to 4 times less than those required by the radio-wave data. Reflections from several of these closely spaced irregularities should give rise to signals comparable with the observations. It appears that the fine structure in these profiles is real and has much smaller vertical dimension than any other published measurements of mesospheric parameters.