The trace metal composition of foraminiferal shells preserved in deep-sea sediments is a critical source of data used in reconstructing the physical and chemical characteristics of ancient oceans. However, paleoceanographic information derived from these tracers is limited by the accuracy with which calcite shells record ambient seawater chemistry and the question of whether changing environmental conditions or interspecific differences affect shell chemistry. Experiments with living foraminifera are one of the most direct ways to quantitatively evaluate the relationship between shell and seawater chemistry and the influence, if any, of environmental variables. Here we present results from laboratory experiments on the living planktonic foraminifers Globigerina sacculifer and Orbulina universa that demonstrate that precipitated shells provide an accurate record of seawater barium, a tracer used in reconstructing nutrient and alkalinity distributions in past oceans. On the basis of culturing data at 29 ¿C, 36.7%0 salinity, the best estimate of the partition coefficient D relating foraminifera shell Ba/Ca to seawater Ba/Ca is 0.147¿0.004. Data show that when environmental conditions are held constant, there is no significant variability in Ba uptake between individual shells of one or both species. Results also demonstrate that temperature varying over a 7 ¿C range and salinity varying over a 3%0 range do not appreciably influence Ba uptake.