The relationship between shell size and Mg/Ca, Sr/Ca, δ18O, and δ13C of planktonic foraminiferal calcite has been investigated using seventeen species in six different size fractions. Mg/Ca increases and Sr/Ca decreases with increasing size, except for two globorotaliid species which show the opposite trend. The changes in Mg/Ca broadly follow δ18O calcification temperatures except that surface and near-surface dwelling species show larger changes in Mg/Ca than can be accounted for by differences in calcification temperature derived from δ18O. The increases in Mg/Ca and decreases in Sr/Ca vary linearly with the well-established increase in δ13C with size. This is consistent with smaller individuals calcifying faster than larger individuals and larger individuals forming calcite that more closely reflects seawater temperature and composition. It appears that variations in calcification rate affect Mg and Sr shell chemistry. The observations may account for part of the temporal variability in foraminiferal Sr/Ca within a single size fraction that has been attributed to changes in seawater Sr/Ca.