The productivity of macrophytes has been studied in three different zones of Lake Saint-Pierre, Québec: terrestrial, semiaquatic, and aquatic. From the data collected, six vegetation units can be defined. The productivity (grams per square metre) of each unit is as follows: Calamagrostis canadensis, 528; Phalaris arundinacea, 1096; Sparganium eurycarpum, 508; Scirpusfluviatilis, 436; Scirpus acutus, 284; Vallisneria americana, 138. The vegetation distribution is described as a function of elevation and distance from the shore. The results are compared with available data in the literature.