Microzoning of areas undergoing a particularly high seismic risk is discussed in this paper, with special emphasis on the seismological aspect of the problem. The area of Librino (Catania) was chosen as a test area for this study, in order to extend, in the near future, the method adopted to the whole town of Catania as well as to other seismic areas in Sicily. Microtremors where used as a seismic wave source for the study of the frequency response of different sites. This method has shown a sufficient reliability and gives an economic mean for the investigation of different soil responses to a seismic input. As a result, it was possible to draw a map of resonant frequencies typical of different sites which represent a preliminary map of zonation particularly useful for engineering purposes. Frequency dependent amplification peaks, observed through spectral analysis, were compared with theoretical fundamental period of sites, obtained through the assumption of simple physical models. This gave the opportunity to formulate some hypothesis for the interpretation of results obtained, and can allow the extrapolation of them to similar areas where field measurements of noise cannot be performed.