Laboratory experiments with the planktonic foraminifera Globigerina bulloides (nonsymbiotic) and Orbulina universa (symbiotic) were carried out to examine the effects of temperature, irradiance (symbiont photosynthesis), [CO3/2-], and ontogeny on shell δ13C values. In ambient seawater ([CO3/2-] = 171 μmol kg-1), the δ13C of G. bulloides shells decreases 0.11‰ °C-1, a pattern that likely results from the incorporation of more respired CO2 into shell carbon at higher metabolic rates. The δ13C of O. universa shells grown under low light (LL) levels is insensitive to temperature and records the δ13C value of seawater ΣCO2, whereas the δ13C of high light (HL) shells increases slightly with temperature (0.05‰ °C-1). HL O. universa grown in elevated [CO3/2-] seawater are isotopically depleted relative to those grown in ambient seawater, although it is uncertain from these experiments whether the [CO3/2-] influence on δ13C is affected by temperature. When applied to deep-sea core material, these results demonstrate that differences in sea surface temperature and [CO3/2-] can bias how we interpret downcore shifts in foraminiferal δ13C. (C) 2000 Elsevier Science B.V.