Abstract.The composition of the algal diet ofSiganus luriduswas compared with that of the algal vegetation found in fish habitats along the Mediterranean coast of Israel. North‐south differences in the diet were found and reflected differences in the available algal resources as well as in algal species selected.In the northern area, the diet and the algal selection differed considerably betweenS. luridusand a congeneric Lessepsian migrant siganid.S. rivulatus, whereas in the southern area the two species showed similarity in feeding.A comparison is made between the Mediterranean populations and those of the tropical Red Sea Gulf of Elat (Aqaba). The diet partitioning in the Northern Mediterranean resembles that found in the Gulf of Elat. where the realized diet may be related to strong competition between herbivores in combination with availability of numerous edible algal species. There. the two siganids showed different preferences. In the Northern Mediterranean, competition is probably lower. since there are fewer herbivorous fish species. while numerous good algal habitats (rocky areas) enable selection. By contrast, rocky algal habitats are scarce in the southern area; this implies that the total amount of algae available to choose from is smaller there.The similar diets and preferences in the southern part of the Mediterranean indicate scarcity of preferred food species. as the fishes fed mainly on small filamentous species of the algal turf on the bottoms.SummaryThe algae and seagrasses in gut contents of two Lessepsian migrants were analysed:Siganus rivulatusandS. luridusfrom both Red Sea and eastern Mediterranean locations. The quantitative and qualitative composition of the diet was compared to that of the algal and seagrass vegetation in four regions: the southern and northern basins of the Gulf of Elat and the southern and northern area in the Mediterranean along Israel's coast.In the original environment of the fish in the southern basin of the Gulf of Elat, a variety of large brown algae as well as fleshy and soft red algae, green and blue‐green algae and seagrasses inhabited the coral reefs and adjoining areas, all of which were accessible to the fish. There,S. luridusmainly fed on and selected large brown algae, whileS. rivulatusmainly fed on and selected fleshy and soft red algae. In the Mediterranean, horizontal platforms at about sea level are covered with algae. The small tidal range makes these sites mostly inaccessible to grazing fish. In the submerged habitats where the fish fed, calcareous red algae dominated. In the southern area both siganids fed on and selected small filamentous algae as found in the algal turf. In the northern area S.luridusfed on and selected large brown algae,S. rivulatusonUlvaand fleshy red algae. The algal partitioning in the Red Sea disappeared in the southern area of the Mediterranean as an adaption to the environment. It developed in the northern area probably due to the prevalent rocky areas with their algal habit