We have measured the isotopic fractionation of zinc (Zn) during uptake by the marine diatom Thalassiosira oceanica cultured at a range of free Zn super(2+) concentrations representative of the natural range from coastal and oligotrophic regions of the ocean. Harvested cells were rinsed with either plain seawater or a wash designed to remove adsorbed extracellular metals. Unwashed cells had much higher levels of Zn and were isotopically heavier than the media, indicating a positive isotope effect for extracellular Zn adsorption. Internalized Zn, measured in washed cells, was isotopically lighter than the media. The magnitude of Zn isotope fractionation changed with free Zn2+ concentration, corresponding to a switch on the part of T. oceanica between the predominance of high- and low-affinity Zn transport pathways. The total isotope effect for uptake (d super(66)Zn) was -0.2% for high-affinity uptake at low Zn concentrations and -0.8% at the highest Zn concentrations, where low-affinity uptake is dominant. To our knowledge, this is the first study to describe a physiological basis for biological metal isotope fractionation during transport across the cell membrane. Similar high- and low-affinity Zn transport pathways are common among marine phytoplankton, suggesting that the processes described here are an important factor in natural marine Zn isotope variations.