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In this thesis, the issue of the incorrect indication of the species in fish with the trademark "galeos" was investigated using the gene markers 5S rDNA, 16S rDNA and COI. For this purpose 30 samples, with the trademark "tope", were collected from seven different open markets and three fish markets of Thessaloniki. Thereafter, DNA extraction from the muscle tissue of fish was performed and DNA control was conducted in agarose gel, with the use of ultraviolet radiation. Subsequent gene amplification of 5S rDNA, 16S rDNA and COI by the PCR method, and control of the PCR products on agarose gel were held. Based on PCR products, the gene 5S rDNA proved to be unsuitable marker for species identification, as it does not present differences of the PCR product among the samples. Thus, the identification of the samples was performed with sequencing analysis of mitochondrial genes 16S rDNA and COI, and using the BLASTn tool on GenBank and the Identification System on BOLD. Finally, it was found that only twelve of the thirty samples were classified in the Mustelus spp species, which according to the legislation corresponds to the scientific name “galeos”. It is worth mentioning that all the fish markets sell the authentic “galeos” species, in contrast to the 20% of the open markets.