Abstract: Marine nanoplanktonic diatoms have been frequently either underestimated or overlooked in phyto-plankton investigations due to their small size (2–20 μm), which allows most of them to pass through a regular phytoplankton net of 70-μm pore size. For nanoplanktonic diatom classification, water samples (each 2000 mL) were collected from several representative sea areas of Chinese coast from May 2002 to May 2006. Pelagodictyon Clarke was identified as a newly recorded nanodiatom genus for China under transmission electron microscope (TEM). This genus is characterized by the delicate structure of the valve face. The striae are seen to be formed by ribs with cross-ribs (frets) and the intervening spaces are occupied by polygonal cribra situated much close to the exterior face of the valve. Just one species was found, i.e., P. tenue Clarke. Detailed description of the taxonomic characteristics with TEM photographs of this species and its ecological habitat and distribution are given in this paper. Morphological characteristics comparisons among several resembling genera were discussed.

Key words: China, fret, nanodiatom, new record, Pelagodictyon, Stephanodiscaceae