The effect of copper (Cu) on the daily growth rate (DGR) and photosynthetic efficiency (PE) of the brown macroalga Padina boergesenii was studied under controlled laboratory conditions. The DGR was measured as an increase in fresh weight while the PE was measured as the ratio of variable to maximal chlorophyll fluorescence (Fv /Fm ). Padina boegesenii were exposed to 25, 50, 100 and 500mg Cu/l for 21 days. The DGR and PE of P. boergesenii decreased significantly with an increase in copper concentration in the growth media. This study shows that copper is acutely toxic to P. boergesenii at concentrations higher than 500mg Cu/l. Low concentration of copper is toxic to P. boergesenii after a prolonged exposure of 21 days. Similarly, exposure time has a significant negative effect on both DGR and PE.