Carbon and oxygen isotope fractionation in two morphotypes (Type I and Type II) of the symbiont bearing planktic foraminifer Globigerinella siphonifera (d'Orbigny) is investigated. SCUBA collected specimens were grown in the laboratory under identical culture conditions and their stable isotope signature was analyzed to characterize the influence of ontogeny and feeding rate on their d13C and d18O signals. The two Types show a positive linear correlation between d13C and d18O with size. Type II is enriched in 13C and 18O relative to Type I and the enrichment per size increment is greater than for Type I. The carbon isotope composition of Type I tends towards lighter values at higher feeding rates whereas Type II is unaffected by the feeding regime.In order to determine if the isotopic response can be attributed to differences in growth characteristics and/or host/symbiont interactions, specimens were cultured under a variety of conditions and the pigment composition of freshly collected specimens was measured. Type I has a much lower photo-pigment content, which probably implies a lower gross photosynthetic rate. In addition, its growth- and calcification- rate are lower. The impact of these life processes on the stable isotope composition is discussed and it is argued that isotope fractionation is controlled by two linked processes. The carbon isotope fractionation is affected directly by a 12C depletion or enrichment of the microenvironment via symbiont photosynthesis and host respiration respectively. Concurrently, the life processes invoke a kinetic fractionation of the carbon and oxygen isotopes via their impact on the ambient carbonate chemistry.