We studied the interactive effects of iron (Fe) and copper (Cu) availability on the growth rates, Cu quotas, and steady-state Cu-uptake rates (ρ ssCu) of 12 phytoplankton (from four classes and two marine environments). A mixed-effect statistical model indicated that low Fe significantly decreased phytoplankton growth rates. In contrast, lowering Cu levels only decreased the growth rates of the oceanic phytoplankton. Under Fe/Cu sufficiency, the Cu quotas ranged from 0.36 to 3.8μmol Cu · mol -1 C. Copper levels in the growth medium had a significant positive effect on the Cu quotas, and this effect was dependent on the algal class. Under Fe/Cu sufficiency, the highest average Cu quotas were observed for the Bacillariophyceae, followed by the Cyanophyceae, Prymnesiophyceae, and lastly the Dinophyceae. Similar taxonomic trends were observed for the ρ ssCu. Although the Cu:C ratios were not significantly higher in oceanic strains, there are five independent lines of evidence supporting a more important role of Cu in the physiology of the oceanic phytoplankton. The mixed-effect model indicated a significant Cu effect on the growth rates and ρ ssCu of the oceanic strains, but not the coastal strains. In addition, lowering the Cu concentration in the media decreased the Cu quotas and ρ ssCu of the oceanic strains to a greater extent (5.5- and 5.4-fold, respectively) than those of the coastals (3.8- and 4.7-fold, respectively). Iron limitation only had a significant effect on the Cu quotas of the oceanic strains, and this effect was dependent on Cu level and taxonomic class. Our results highlight a complex physiological interaction between Fe and Cu in marine phytoplankton. © 2012 Phycological Society of America.