The spectral ratio technique of Phinney, R. A., 1964: Structure of the earth's crust from spectral behavior of long period body waves (Journal of Geophysical Research 69, 29973017) has been used for estimating crustal thickness beneath 11 Fennoscandian seismograph stations equipped with three-component long period instruments. Extended areas of Mono depths of the order of 45 km were found in the northeastern parts of Fennoscandia with a local maximum of 47 km in the Bothnian Bay (northern Gulf of Bothnia). The coastal areas of southeastern Norway and Zealand (Denmark) exhibit crustal thicknesses in the range 28–33 km. These results agree well with those obtained from conventional seismic profiling when the length of the profile is 300 km or more. A comparison between our results and other geophysical data gives that the oldest provinces exhibit the thickest crust and the lowest heat-flow rate as well. The correlation between Moho depths and geophysical parameters such as post-glacial uplift and gravity anomalies appears at best to be poor.