A comparative study of copper (Cu) toxicity and tolerance in two species of Ulva from Korea, the native Ulva pertusa and alien Ulva armoricana, was conducted by examining the effects on growth, pigmentation, chlorophyll fluorescence, antioxidant capacity and nitrate reductase activity. Toxic effects of Cu were less expressed in U. armoricana than in U. pertusa. At lower concentrations (25-50 microgL(-1)), exposure to Cu did not affect thallus growth of U. armoricana, whilst growth was significantly reduced in U. pertusa. An increase in chlorophyll concentrations was observed in U. armoricana exposed up to 100 microgL(-1), whereas Cu caused a significant chlorophyll reduction in U. pertusa. Chlorophyll b was reduced to a lesser extent than chlorophyll a by higher Cu concentrations. In U. armoricana, the maximum efficiency of photosystem II, minimum fluorescence, maximum electron transport rate and non-photochemical quenching were unaffected by Cu except at the highest concentration tested. U. pertusa showed a significant decrease in those parameters at much lower Cu concentrations. It was notable that in this alga the maximum efficiency of photosystem II was reduced at higher Cu concentrations than relative electron transport rate. Elevated concentrations of Cu induced a strong activation of antioxidant activity in U. armoricana, whereas the generation of high levels of reactive oxygen species probably decreased the non-enzymatic antioxidant defense system in U. pertusa. An increase in the nitrate reductase activity of U. armoricana at 50-100 microgL(-1) Cu coincided with the increase in chlorophyll contents, whereas U. pertusa showed a significant decrease at the higher Cu concentration. Differences in the sensitivity of the two species of Ulva to Cu may influence their competitive interactions in Korean coastal waters experiencing temporal increases in the loading of heavy metals.