Growth and development of fast ice and underlying platelet layer were studied in Atka Bay, Weddell Sea (70°35.05'S, 8°.41'W) from June to December 1995. Cores taken fortnightly were analysed for structural and geochemical properties as well as diatom enumeration. The light regime above and under the ice was measured continuously during the sampling period.The fast ice attained 2 meters thickness by November at an average growth rate of 1cm day-1. More than 60% of the ice consisted of consolidated ice platelets that had accrued under the ice from July onwards. Below this a loose layer of platelets accumulated to 1.50m thickness. Maximum snow cover was 20 cm.Temporal changes in sea ice structure and growth, biogeochemistry and the light regime result in distinct differences between the diatom assemblages in this habitat, and give rise to 4 clear groupings. Maximum standing stock of diatoms was 41mg chla m-2 which is within the range of 20 to 70 mg determined in other studies. These results confirm that fast ice around Antarctica has very characteristic structural and biological features which differentiates it from other sea ice and accounts for its unique role and ecological significance along a large portion of the Antarctic coastline.