ABSTRACT: The calcareous algae are widespread in the early Eocene, shallow marine subsurface Sylhet Limestone Formation of the Bengal Basin, India. The study of these limestones revealed the presence of 07 Halimedacean algal species such as Halimeda incrassata (Ellis) Lamouroux, Halimeda tuna (Ellis and Solander) Lamouroux, Halimeda cylindracea Decaisne, Halimeda nana Pia, Halimeda simulans Howe, Halimeda opuntia (Decaisne) Agardh and Halimeda sp. and 03 Udoteacean algal species such as Ovulites pyriformis Schwager,Ovulites arabica (Pfender) Massieux and Ovulitesmargaritula Lamarck for the first time. The possible reason for the prolific growth of the Halimeda in the early Eocene carbonates of the Bengal Basin may be the nutrients brought by the then river runoff and the currents existed along the shelf. The dominance of halimedacean and udoteacean algae associated with dasycladaleans indicate that the early Eocene Sylhet Limestone of the Bengal Basin was deposited under shallow, warm, tropical, open lagoonal to shelf environment at the depth of 5-6 m below low tide level with Tethyan affinity.