The origin of cooperatively melting regions in DNA, which appear as fine structures in the optical melting profile, has been examined for DNA fragments of known base sequences from bacteriophages phiX174 and fd. Thermal stability maps, which indicate the states of base pairs along these DNA strands, were constructed within the established theoretical framework using the parameters which best reproduce the melting profiles obtained by high temperature resolution experiments. By comparing these stability maps with genetic maps, it was found that several cooperatively melting regions which span several hundred bases have some correlation with the gene locations.