Modern planktonic foraminifera are known to be depth stratified within the water column, their calcite tests often reflecting the oxygen and carbon isotopic compositional differences related to depth differences. I have determined the relative depth rankings of 13 planktonic foraminiferal species of Miocene age preserved in the western equatorial Pacific at DSDP Site 289. A total of approximately 600 analyses of oxygen and carbon isotopic composition were made The depth rankings inferred from the oxygen isotopic data do not agree with those from the carbon isotopic data for all species or for all time intervals. The inferred shallow-dwelling, spinose species (Globigerinoides immaturus, Globigerinoides sacculifer, Globigerinoides obliquus, Dentoglobigerina altispira and Globoquadrina baroemoenensis) were likely hosts to large concentrations of algal symbionts and preserve delta (super 13) C values that are inferred to have been fractionated through the metabolic process of algal symbiont photosynthesis. From approximately 18.7 to 7.5 Ma, Globigerinoides obliquus, Globigerinoides sacculifer, Globorotalia (Jenkinsella) siakensis, Globorotalia (Jenkinsella) mayeri, Dentoglobigerina altispira, Globoquadrina baroemoenensis and Globigerinoides immaturus were shallow-dwelling forms, and Globoquadrina dehiscens, Globorotalia (Menardella) limbata, Globorotalia (Menardella) menardii, Globoquadrina venezuelana and Globorotalia (Menardella) praemenardii were deep-dwelling forms. From approximately 7.5 to 5.8 Ma, shallow-dwelling forms included: Globigerinoides obliquus, Globigerinoides sacculifer, Dentoglobigerina altispira, Globoquadrina baroemoenensis, Globorotalia (Menardella) limbata and Globigerinoides immaturus, intermediate-dwelling forms included: Globorotalia (Menardella) menardii and Globorotalia (Globorotalia) merotumida, and deepdwelling forms included: Globoquadrina venezuelana. The evolution of the vertical thermal structure and its stability in this region was largely in response to major climatic and cryospheric changes at high latitudes and the closure of the Indonesian Seaway. Comparisons of the composite oxygen isotopic curves each representing the surface mixed layer, the thermocline and deep waters reveal three distinct periods of thermocline stability during the Miocene ( approximately 18.7 to 16.1 Ma, approximately 11.1 to 9.9 Ma and approximately 7.5 to 5.8 Ma) separated by two intervals of thermocline instability ( approximately 16.1 to 11.1 Ma and approximately 9.9 to 7.5 Ma). Between approximately 18.7 and 5.8 Ma, surface waters warmed 2 to 4 degrees C and deep waters cooled 2 to 4 degrees C.