Electrophoretic method has been applied to biochemical classification of Mugilids in the Lagoons of the Po delta. Among the various proteins analysed the one which turned out to be the most useful was the lactate dehydrogenase enzyme (LDH) because it presents different patterns in the five species living in the above mentioned lagoon and does not show any variations in the course of ontogenesis. This caracteristic makes it possible to identify stocks of fry to be put into closed waters for subsequent cultivation. Moreover this method allows one to make certain phylogenetic speculations with regard to the origin of these species.