A taxonomic and ecological study of the phytoplankton of the estuaries of the Schelde (Belgium/The Netherlands) and the Elbe (Germany) revealed the presence of numerous species belonging to the diatom genus Thalassiosira. Its representatives were a dominant component of the phytoplankton spring bloom in these estuaries. A total of 13 species were identified. All of these were present in the Schelde, while only 10 were found in the Elbe. Seven and five species respectively had previously not been reported from the Schelde and Elbe. A detailed light and electron microscopical description of these species is given.