The effeet of copper sulphate on morphology and physiology of Lyngbya nigra has been studied. The growth was inhibited in all treatments (0.4 to 80.0 ^tM) of eopper sulphate. There were no apparent morphologieal changes up to 0.8 fiM and during the first two days of treatment even in the higher eoneentrations of eopper sulphate.In eoneentrations above 0.8 ixM the first symptom of toxicity was the formation of separation discs in large numbers. The trichomes contracted longitudinally and the cells became swollen and constricted at the cross walls. The eells also beeame yellowish due to loss of photosynthetie pigments. Finally, in 4 /iM and above, vaeuoles appeared in large number indicating the moribund state of the eells.Copper sulphate inereased respiration at 2 fiM, and optimum effeet was observed in 8 nM after 96 h. Inhibition of photosynthesis was deteetable in 0,8 fiM, and 100% inhibition took plaee in 8 ixM after 96 h. In higher eoneentrations the effeet was immediate, and a conspicuous inhibition of photosynthesis couid be observed within 10 min.The copper content of the alga increased with increased concentration of copper sulphate while potassium content decreased. With rise in outside concentrations of copper, there was a comparatively great increase of absorption in 2 and 4 fiM, while further increases were gradually less. The observations indicate that changes in the physiological activity of the alga under treatment are closely interlinked with marked changes in morphology.