It has been studied long term effects of copper upon physiological processes and growth ofChlorella saccharophila(Krüger) Migula andCyanidium caldariumGeitler, two algae which have the optimum of pH at 6,3 and 1,5 respectively. The algae differed markedly in their resistance to the copper. Infact, growth ofChlorella saccharophilashowed a lag time in presence of sublethal copper concentrations (0,25-1 mg/l). By contrast,Cyanidium caldariumwas about 4000 fold more tolerant to the metal. In both algae photosynthesis was more sensitive to applied copper than was respiration.