The effect of the continuous addition of copper to a small calcareous stream at a concentration of approximately 120 μg/litre was studied for a year. Of the parameters evaluated in this study, the species composition of the periphyton was found to be the most sensitive and informative measure of the effects of copper. Two of the dominant species of algae in the stream were eliminated from the periphyton: the diatom Cocconeis placentula var. euglypta, and the filamentous green alga, Cladophera glomerata. Cocconeis placentula, which commonly contributed 85 to 98 percent of the summer diatoms in the treated reach of the stream prior to the addition of copper, was replaced by three species of diatoms, Nitzschia palea, Navicula nigrii, and N. seminulum var. hustedtii. Other species of algae that were more abundant in the copper-treated reach than in the control reach of the stream were the filamentous blue-green alga, Schizothrix calcicola, and the desmids, Cosmarium granatum and C. subprotumidum.