The concentration of metals by marine phytoplankton, assessed for diverse species in laboratory culture experiments with radiotracer methodology and taken together with literature values for other metals, was analyzed in light of geochemical models describing particle surface chemistry. Concentration factors vary among the metals from -0 to = 106, and there are only modest differences (less than an order of magnitude) among algal species for any particular metal. Regression analyses show that, at equilibrium, the logs of the concentration factors are exponentially related (P < 0.001) to solubility products of metal hydroxides and to cytotoxicity and are linearly related (P < 0.001) to the log of the mean oceanic residence times (years) of the metals. It would appear that concentration factors and toxicity of metals in marine phytoplankton and oceanic residence times of metals can be predicted to within an order of magnitude from the chemical literature. The flux of particulate matter in marine and freshwater systems has been recognized as a major factor in regulating metal concentrations in natural waters (Sigg et al. 1982; Santschi 1984). Since phytoplankton and planktonic debris are major components of natural particulate matter, they can play significant roles in mediating the vertical transport of metals (particularly out of surface waters), consequently affecting oceanic residence times of the metals. It is of interest therefore to assess the accumulation of metals by phytoplankton in the context of geochemical cycling of metals, as well as of toxicity and nutritional considerations. The degree to which metals are concentrated out of seawater by phytoplankton is a function of their speciation in water and their affinity for the available surface ligands (Davies 1978). Metal affinity for various ligands, including hydroxides, is correlated with the polarizing power and the charge density of the metals (Turner et al.