The Mediterranean toothcarp Aphanius fasciatus (Valenciennes, 1821) is a small-sized omnivorous estuarine fish. Its diet is dominated by juveniles of shrimps (Palaemon adspersus), Isopods, Branchiopoda, Bivalvia, eggs of invertebrates, mosquitoes (adults and larvae) and diatoms. An ontogenetic diet shift with an increase in mean prey size with fish length was observed. Smaller fish feed on planktonic prey (e.g. copepods, ostracods, nauplii of Artemia), while larger fish prefer larger and more benthic preys (e.g. ampipods, Bivalvia). The diet of A. fasciatus shows a high degree of seasonal variation, with a reduction in the feeding activity during the periods of adverse environmental conditions (winter and autumn). It is a well-adapted estuarine fish, its feeding mode and preferences depending on the preys that are available. Its feeding strategy is characterised by specialisation in different resource types (aquatic invertebrates and mosquitoes) and a high between-phenotype contribution (BPC) to niche width, with specialised individuals showing little or no overlap in resource use.