Cultured planktonic foraminifera, Orbulina universa (symbiotic) and Globigerina bulloides (nonsymbiotic), are used to reexamine temperature: Δ18O relationships at 15¿--25 ¿C. Relationships for both species can be described by linear equations. Equations for O. universa grown under low light (LL) and high light (HL) share a slope of -4.80 (0.21? ¿C-1) with a HL-LL offset of -0.33? due to symbiont photosynthetic activity. The effect of <CO32-> on O. universa is -0.002? μmol-1 kg-1 and is insensitive to temperature. For G. bulloides, ontogenetic effects produce size-related trends in temperature: Δ18O, whereby larger shells are enriched in 18O relative to smaller specimens. The O. universa temperature: Δ18O relationships are more accurate than previously published equations for describing plankton tow data. Our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic Cibicidoides data below 10 ¿C. Temperature: Δ18O relationships for G. bulloides provide good agreement with field data for this species from the northeast Pacific. ¿ 1998 American Geophysical Union