The stable carbon isotopic composition of particulate organic matter in the ocean, δ13C(POC), shows characteristic spatial variations with high values in low latitudes and low values in high latitudes. The lowest δ13C(POC) values (-32‰ to -35‰) have been reported in the Southern Ocean, whereas in arctic and subarctic regions δ13C(POC) values do not drop below -27‰. This interhemispheric asymmetry is still unexplained. Global gradients in δ13C(POC) are much greater than in δ13C(DIC), suggesting that variations in isotopic fractionation during organic matter production are primarily responsible for the observed range in δ13C(POC). Understanding the factors that control isotope variability is a prerequisite when applying δ13C(POC) to the study of marine carbon biogeochemistry. The present model study attempts to reproduce the δ13C(POC) distribution pattern in the ocean. The three-dimensional (3D) Hamburg Model of the Oceanic Carbon Cycle version 3.1 (HAMOCC3.1) was combined with two different parametrizations of the biological fractionation of stable carbon isotopes. In the first parametrization, it is assumed that the isotopic fractionation between CO2 in seawater and the organic material produced by algae, ε(p), is a function of the ambient CO2 concentration. The two parameters of this function are derived from observations and are not based on an assumption of any specific mechanism. Thus, this parametrization is purely empirical. The second parametrization is based on fractionation models for microalgae. It is supported by several laboratory experiments. Here the fractionation, ε(p), depends on the CO2 concentration in seawater and on the (instantaneous) growth rates, μ(i), of the phytoplankton. In the Atlantic Ocean, where most field data are available, both parametrizations reproduce the latitudinal variability of the mean δ13C(POC) distribution. The interhemispheric asymmetry of δ13C(POC) can mostly be attributed to the interhemispheric asymmetry of CO2 concentration in the water. However, the strong seasonal variations of δ13C(POC) as reported by several authors, can only be explained by a growth rate-dependent fractionation, which reflects variations in the cellular carbon demand. (C) 2000 Elsevier Science B.V.