Ege University, Faculty of Science, Biology Department, İzmir-Turkey In this investigation, an evaluation has been made of the importance of the marine flora of Izmir Bay in the biodiversity of the Turkish Aegean and Mediterranean, and of environmental factors threatening these coasts and affecting the phytobenthos in recent years. An assessment has been made of the big role played by Cyanophyta (102 taxa), Rhodophyta (416 taxa), Heterokontophyta (146 taxa) Chlorophyta (151 taxa) and Tracheophyta (5 taxa) in the biodiversity of the Turkish Aegean and Mediterranean. The flora of the Turkish Mediterranean and Aegean Sea coast includes total 820 taxa, which five are flowering plants. 21 taxa (18 algae and two flowering plants) are threatened or endangered. This number could rise on various localities. Threatened or endangered seagrass habitats include Posidonia oceanica (L.) Delile, Zostera marina L., Z. noltii Horneman, Cymodocea nodosa (Ucria) Ascherson beds and Lithophyllum tortuosum (Esper) Foslie f. tortuosum, Mesophyllum lichenoides (Ellis) Lemoine and Dictyopteris membranacea (Stackhouse) Batters algal communities. Keywords : AEGEAN, ALGAE, BIODIVERSITY, MEDITERRANEAN, TURKEY, SEAGRASSES