The Mg/Ca and oxygen isotope ratios (δ18O) of multiple species of planktic foraminifers provide information on the hydrological conditions between the surface and the thermocline. Knowledge of the apparent calcification depth (ACD) of planktic foraminifers is key to reconstructing paleoenvironments; however, ACDs exhibit seasonal variations and differ over regional scales. We obtained the ACDs of Globigerinoides ruber, Trilobatus sacculifer, and Neogloboquadrina dutertrei in the Bay of Bengal using multiyear sediment-trap samples collected at approximately 900 m depth. The sediment traps were moored in the southwestern Bay of Bengal, with sampling intervals of 17-42 days. The temperature estimates obtained from the δ18O and Mg/Ca patterns of G. ruber, T. sacculifer, and N. dutertrei indicate that G. ruber reflects the temperature within the mixed layer, whereas N. dutertrei precipitates its test in the upper thermocline and T. sacculifer calcifies between these depths. The rapidly attenuating photosynthetically active radiation constrains the living depths of these symbiont-bearing species to within the upper 60 m of the euphotic zone in the southwestern Bay of Bengal. Although G. ruber and N. dutertrei calcify at different depths, as demonstrated by the different δ18O values of the two species (Δ18Or-d), large Δ18Or-d values were not obtained just in spring and summer when stratification is developed . The flux-weighted δ18O value of a species corresponds to the mean annual δ18O value of that species. Seasonal variations in species-specific test fluxes can be averaged out because of recurring flux peaks during the northeast and southwest monsoon seasons.