AbstractA laboratory investigation was conducted to study the effect of various concentrations of copper on the freshwater algaeScenedesmus quadricaudaandChlorella vulgaris. Electron‐dense inclusions were observed in the vacuoles ofS. quadricauda, and this alga showed some growth at a dose as high as 2,500 μg.1−1. The occurrence of the electron‐dense inclusions was correlated significantly with the copper concentration and seems to be implicated in the tolerance of the species.C. vulgariswas much more sensitive to copper thanS. quadricaudaand showed osmotic changes and membrane