This study is aimed at the immigrant fish species along the 2000 km of the coast of Libya, focusing on Lessepsian migrants. Sixteen Lessepsian fish species, four of them new for Libya, have recently been found. Their distribution and characteristics have been studied. Six of them are of commercial value, two of them, Siganus luridus and S. rivulatus, were selected for detailed biological and ecological studies. They are well established and successful breeders in the coastal waters, even though the temperature range and habitats differ considerably in comparison with the original area of distribution, the Red Sea. Both species show very similar, generalist food preferences. From the view of genetics, using mitochondrial DNA, the results suggest that both species (Siganus spp.) of Red and Mediterranean Seas belong to the same gene pool, and assuming the migration in the Mediterranean they took two pathways, to the north-west and south-west.