The majority of microalgae living in connection with sea ice are found at the bottom of the ice. Here they live either as a loosely attached thin layer on the ice underside, as in the pack ice of the Barents Sea, or in the bottom 5–10 cm of the fast ice in the Arctic and in Antarctica. For those algae living inside the ice or in the brine the environment may provide quite extreme conditions. There are different algal species inhabiting different parts of the ice and the brine, and they are all different from what is found in the water masses below the ice. Temperature and salinities change from top to bottom in the ice cover and when going from ice to sea water. Light intensities differ greatly from top to bottom, particularly if there is snow on top of the ice, and together these factors structure the presence of ice algae in the ice. The presentation will discuss environmental factors and the algae found in the fast ice of the Ross Sea in Antarctica in spring/early summer of 2009.