Recent work has provided useful Mg/Ca to water temperature calibrations for shallow-dwelling planktonic foraminifer species. Globorotalia truncatulinoides (right coiling (R)) is a deep-dwelling species that can serve as a source of information about the temporal variability in the water characteristics of the thermocline. We present a temperature calibration for the Mg/Ca in the shell of G. truncatulinoides (R) and examine some of the practical issues associated with evaluating the usefulness of the technique. The Mg/Ca in the primary and the secondary calcite of individual G. truncatulinoides (R) correlates exponentially with water column temperatures, showing a change of ~10% in the Mg/Ca per 1¿C (R2 = 0.92). A limited comparison with plankton tow samples demonstrates that the average Mg/Ca temperature was offset +1¿C from the average temperature calculated using the δ18O calibration of O'Neil et al. <1969>, and the Mg/Ca temperatures have a range similar to the δ18O temperatures. Comparisons of the in the core top samples to water depth of deposition indicates that dissolution does not alter the measured value of Mg in the primary calcite.