Stable carbon isotope fractionation by seventeen species of marine phytoplankton, representing the classes of Bacillariophyceae, Chlorophyceae, Prasinophyceae, Chrysophyceae, Haptophyceae and Dinophyceae have been determined in laboratory culture experiments using bicarbonate enriched artificial sea water. The ∆ HCO 3- values (∆ HCO 3- = δ 13C of algae vs HCO 3-) range from -22.1 to -35.5%. Nitzschia closterium shows the smallest fractionation of - 22.1% and Isochrysis galbana, the greatest of -35.5%,. Since these algae were cultured under identical laboratory conditions, the wide range of ∆ HCO 3- values is seemingly due to the presence of different metabolic pathways within these organisms. A temperature dependent fractionation of 0.36% per °C with decreasing temperatures was measured for Skeletonema costatum whereas, smaller temperature dependencies of -0.13, +0.15 and -0.07%. per °C were observed for Dunaliella sp., Monochrysis lutheri and Glenodinium foliaceum, respectively. The consistency of ∆ HCO 3- values of Skeletonema costatum, Dunaliella sp. and Monochrysis lutheri grown at salinities of 22, 26, 32 and 36% indicates that natural salinity variations have negligible effects on the isotopic composition of marine phytoplankton.