The phytoplankton and physicochemistry of the canalized lower Scarpe River in northern France were studied from April 1992 to October 1993. The phytoplankton was dominated by diatoms in spring and autumn and by chlorophytes in summer. There was an irregular occurrence of cyanophytes, which characterized a eutrophic river environment. Certain centric diatoms typical of eutrophic environments (Stephanodiscus hantzschii, Cyclostephanos dubius), which were present in a high proportion in 1992, were replaced in 1993 by other species characteristic of moderate to large size rivers (Asterionella formosa, Aulacoseira granulata). The seasonal variation observed on the lower Scarpe River was similar to that of the large European rivers. Changes in certain chemicals and the phytoplankton community indicated some improvement in river water quality subsequent to the start-up of a new sewage treatment facility in 1992.