The growth of Fucus vesiculosus L. germlings in chemically defined culture media containing a range of Cu concentrations (20–1000 nM) was monitored simultaneously with measurement of the Cu speciation in the media by competitive equilibrium-adsorptive cathodic stripping voltammetry. Fucus vesiculosus germlings were found to exude Cu-complexing ligands with conditional stability constants of the order of 1.6 × 1011. Ligand concentrations increased with increasing total dissolved Cu concentrations (CuT) until a concentration of 500–800 neq Cu·L−1 was reached. Concentrations of the ligand exceeded CuT in treatments containing 20 and 100 nM Cu, were similar to CuT in the 500-nM Cu treatment, but were less than CuT in the 1000-nM treatment. Therefore, [Cu2+] were calculated to be at concentrations of 10−11− 10−10 M in the 20- and 100-nM treatments, 10−9 M in the 500-nM treatment, and 10−7 M in the 1000-nM treatment. Growth rates were lowest at Cu2+ concentration > 10−9. These results are discussed within the context of the potential roles for exuded copper-complexing ligands.