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The blue shark Prionace glauca (Linnaeus, 1758) is a pelagic fish, belonging to the Carcharhinidae family. It is one of the top predators placed at the top of the food chain and for this reason it must be followed to preserve it against any illegal fishing. Its presence in the landings of the Moroccan fleet by both longliners and boaters shows that this species has become highly exploited and needs to be followed. The main objective of this work is to know its place in the ecological niche determining the ecological role of this predator in the ecosystem. The study of Prionace glauca diet is intended to highlight, in the short term, feeding periods and prey preferences according to the state of maturity of the animal and will allow in the medium term to understand the behavior of this species in the marine ecosystem. Since February 2015, biological sampling has been collected at the wholesale fish market of Casablanca where the catches of Prionace glauca transit from several Moroccan ports. Investigations included size measurements, sex determination and stomach recovery for macroscopic analysis. The frequency of occurrence showed that European pilchard Sardina pilchardus is the main prey in Prionace glauca. The immature individuals eating preference is the European pilchard Sardina pilchardus although for mature are rays and sharks. The cephalopods and rays are more consumed by females whereas Birdbeak dogfish Deania calceus and the swordfish Xiphias gladius by males. The emptiness rate show that 56% of the dissected stomachs were empty indicating that there are periods of low Prionace glauca feeding during the "autumn-winter" seasons.