Quantitative study of climatically sensitive nannofossils in the sediments of the Gulf of Aden has provided a climatic and oceanographic scenario for the northwestern Indian Ocean during the late Neogene. The surface water temperature was relatively cold during the latest middle Miocene to late Miocene (10.7 to 6.32 Ma) with a probable warm interval in the early late Miocene (9.82 to 8.49 Ma). This cooling is attributed to increased upwelling in the northwestern Indian Ocean. The surface water was relatively warm from the latest late Miocene to the middle of the late Pliocene (6.32 to 2.59 Ma). The most intense cooling took place during the late Pliocene to early Pleistocene (2.59 to 1.69 Ma). Intense upwelling during this interval coincides with worldwide climatic deterioration. A probable short warm interval (1.69 to 1.59 Ma) followed this cold interval. In the Pleistocene the surface water was cold from 1.59 to 1.00 Ma and from 0.61 to 0.24 Ma and warm from 1.00 to 0.61 Ma and after 0.24 Ma.