Laser-ablation inductively coupled plasma-mass spectrometry microanalyses of Mg/Ca across individual final chambers of three planktonic foraminifera species, Globigerinoides ruber, G. sacculifer, and Pulleniatina obliquiloculata, reveal significant interspecies differences in test Mg concentrations. Whereas these three species have similar Mg/Ca values at low sea surface temperatures (similar to 22 degrees C), they diverge markedly at high sea surface temperatures (similar to 29 degrees C). Explanations for these differences in species Mg/Ca values based on detailed comparison of species intratest Mg/Ca distributions suggest that compositional variability within tests cannot account for the observed deviation of species Mg/Ca values in warm-water equatorial regions. Multiple regression modeling and delta O-18 analysis of Globigerinoides sacculifer tests indicate that interspecies differences in Mg/Ca values result from different depth habitats. The average Mg/Ca values of G. ruber final chambers reflect the temperature of the surface mixed layer (0-25 m), whereas those of G. sacculifer and Pulleniatina obliquiloculata correlate best with subsurface temperatures at 50-75 m and 100-125 m water depths, respectively. Mg/Ca calibration to the temperatures at these depths reveals a similar temperature control on Mg test composition in all species. Combining our results with Mg/Ca values from published culturing experiments, we derive a generalized equation for the effect of temperature and seawater salinity on foraminiferal Mg/Ca. We also show that the Mg/Ca composition of specific calcite layers within foraminiferal tests, including the low-Mg/Ca layers of Globigerinoides ruber and G. sacculifer and the cortex layer of Pulleniatina obliquiloculata, correlates with seawater temperature and can be used as an additional proxy for seawater temperature.