In 1979 and 1980, batch culture experiments were conducted to observe the inhibitory effect of copper ion (concentrations of 10, 50, 100, 200 and 400 µg Cu .1-1 ) on the standing crops and photosynthesis of phytoplankton of the Saguenay River (for 124 hours) and in Chlorella vulgaris (for 8 days) . These algal assays were carried out using the surface water of the Saguenay River . In natural populatoins of phytoplankton, it was found that photosynthesis was more sensitive than growth : at the lowest concentrations, such as 10 µg Cu . 1 -1 , copper seemed to increase the chlorophyll concentrations whereas the rates of primary production show a decrease of 60% with respect to the control . At higher concentrations of copper, the effect is weak in chlorophyll concentrations and more pronounced in the rates of primary production (decrease of 86 to 90%) . The pennate diatoms are dominant (in all the samples) and these organisms are known as relatively resistant to copper . In Chlorella vulgaris, it was observed that with 100 µg Cu . 1-1 , chlorophyll concentrations and rates of photosynthesis respectively decrease by 63 and 99% with respect to the control . At higher concentrations of copper, a maximum decrease of 70% and 99% respectively for chlorophyll concentrations and rates of primaryproduction are observed .