Records of Sr/Ca changes in planktonic and benthic foraminifera from diverse hydrographic settings reveal coherent variability (5 ± 1%) between ocean basins and between surface and deep waters over the past 300 kyr. There is a general increase in foraminiferal Sr/Ca over the penultimate glaciation declining to minimum values during stage 5 and an increase in Sr/Ca from stage 5 through stage 2. Coincident changes in benthic foraminifera records from the Atlantic and Pacific basins imply that Sr/Ca variations are not dominated by dissolution. Planktonic culturing data provide evidence that the downcore Sr/Ca variations are not controlled by temperature changes and suggest only a small influence of salinity and pH. Variation common to the records is most readily explained by changes in mean ocean Sr/Ca. If fossil foraminifera reliably record higher glacial seawater Sr/Ca, coral Sr paleothermometry would underestimate sea surface temperature during glacialepisodes.