In an aquatic environment, toxicity of metals to organisms depends on external factors (type of metal, exposure concentration and duration, environmental parameters, and water quality) and intracellular processes (metal-binding sites and detoxification). Toxicity of copper (Cu) on the marine microalga Tetraselmis suecica was investigated in this study. Dose-dependent (Cu concentration dependent) inhibition of growth and cell division, as well as, variation of intra- and extra-cellular Cu, Fe and Zn content was observed. T. suecica was sensitive to Cu; the 96 h EC₅₀ (concentration to inhibit growth-rate by 50%) of growth rate (μ) (21.73 μM L^-1), cell division day^-1 (18.39 μM L^-1), and cells mL^-1 (13.25 μM L^-1) demonstrate the toxicity of Cu on this microalga. High intra- (19.86 Pg cell^-1) and extra-cellular (54.73 Pg cell^-1) Cu concentrations were recorded, on exposure to 24.3 and 72.9 μM L^-1of Cu.