O18/O16, C13/C12, and magnesium analyses were performed on a large number of Recent planktonic Foraminifera from South Pacific Ocean sediments. Results show that oxygen isotopic temperatures of Foraminifera tests may be used to locate ocean currents and to define the orientation of large crustal plates relative to the earth's rotational poles. Selective solution effects may cause isotopic temperatures of some species to become progressively colder with increasing water depth of the sediments from which they are taken. Where this is not taken into account, erroneous conclusions may result from the comparison of isotopic temperatures of samples from different locations. Depths at which Foraminifera secrete their tests appear to be determined by density and ultimately by osmotic equilibration with surrounding sea water. Susceptibility of Foraminifera tests to selective solution after death increases with magnesium content. Carbon isotope ratios correlate crudely with both temperature and salinity. The C13/C12 ratio of dissolved or particulate carbon in the oceans is probably the most important factor in determining the C13/C12 ratio of the test.