The effects of copper on the growth, photosynthetic pigments, and antioxidants of the cyanobacterium Nostoc muscorum were studied. Growth behavior under different concentrations of Cu showed strong inhibition with growth rate of 50% reduced at 2.5 µM after 96 h of treatment. There were total inhibitions at 7.5 µM and 10 µM concentrations. Photosynthetic pigments such as chlorophyll-a, phycocyanin and carotenoid contents decreased at the same extent after 96 h of Cu treatment and the inhibition was highest on phycocyanin content. Protein content was inhibited at the same pattern with photosynthetic pigments. Cu-induced lipid peroxidation was concentration and time dependent. Treatment with 10 µM Cu for 2 h resulted in 4.9 folds increase in malondialdehyde (MDA) level in comparison with the control. Similarly, 3.8 folds increased was observed after 5 h treatment with LC50 metal (2.5 µM Cu), compared to the control. The activities of antioxidative enzymes superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) were strongly increased following the increase of Cu concentrations- they reached 159%, 202%, 172% and 179% above the control, respectively.