Changes in the taxonomic composition of diatoms in fast ice as well as in the underlying platelet layer were followed from June December 1995 in Atka Bay, Antarctica. Four communities were clearly distinguished: 1) an interior community dominated by flagellates in spring; 2) a bottom community dominated by a small form of Fragilariopsis cylindrus, 3) a platelet ice layer dominated by Amphiprora kufferathii and Thalassionema sp. growing attached to the ice platelets; and 4) an interstitital community dominated by Chaetoceros neglectus growing in the water between the platelets. Species distribution and succession in these communities were related to changes in silicate concentration, pore space and ice-formation processes and the ability of some cells to grow attached to ice platelets Species composition of platelet layer communities appears to be related to the capacity of some diatoms. The presence of the platelet layer communities may have implications for paleoglaciology.