The Mg/Ca ratio has been measured in two surface-dwelling species of planktonic foraminifera, Globigerinoides ruber and Globigerina bulloides, from biweekly sediment trap samples collected in Guaymas Basin, Gulf of California, between August 1992 and November 1997. The Guaymas Basin experiences significant seasonal changes in sea surface temperature (SST) (~16¿--33¿C) and thus is an ideal location for testing the temperature dependence of Mg incorporation into foraminiferal calcite. The planktonic foraminiferal Mg/Ca ratios are directly compared with concurrent temperature measurements from the study site. The results from the sediment trap study reveal a strong positive correlation between SST and the Mg/Ca ratio in both G. ruber (r2 = 0.86) and G. bulloides (r2 = 0.90). The Mg/Ca ratio increases exponentially by 7% per 1¿C change in temperature for G. ruber and by 6% for G. bulloides. These results indicate that the Mg/Ca ratio in G. ruber and G. bulloides accurately records the measured seasonal surface temperature cycle and interannual variability in the Guaymas Basin within ¿1¿C. The greatest deviation between Mg/Ca--estimated SST and observed SST occurs in the spring and fall for both species. This variability is attributed to rapidly changing hydrographic conditions and possible offset between the time of calcification and the temperature measurements. In addition, G. ruber and G. bulloides exhibit very high Mg/Ca ratios during the strong El Ni¿o--Southern Oscillation years of 1992 and 1997, verifying their potential to record SST up to 33¿C and 28¿C, respectively. The trace metal data are compared with foraminiferal oxygen isotope and alkenone measurements made on the same suite of sediment trap samples to assess the internal consistency of these temperature proxies. Strong correlations exist between the δ18O and Mg/Ca for G. ruber (r2 = 0.86) and G. bulloides (r2 = 0.79). Similarly, sea surface temperature estimates from Mg/Ca ratios are in good agreement with alkenone-derived temperature estimates (r2 = 0.89 for G. ruber and r2 = 0.81 for G. bulloides).