AbstractGrimmia antarcticiis abundant on the Bailey and Clark Peninsulas, Wilkes Land, East Antarctica. It grows as a turf in wet locations and as cushions at relatively dry sites. This paper documents substantial differences in the internal CO2concentration of cushion and turf communities during the growing season. In November, CO2levels in the rhizo/phylloplane of both growth forms were close to ambient (around 350 ppm). Over the next two months the levels increased tenfold in turf but remained essentially constant in cushions. This means that the actively growing shoot tips of turf would be photosynthesizing at optimal CO2concentration (i.e. close to CO2saturation) for most of the summer. Photosynthesis in cushions, on the other hand, would be severely limited by CO2. These findings may account for the observations of Kappenet al.(1989), made at the same field sites in January 1986, that turf photosynthesizes at higher rates than cushions under simulated field conditions and is more resistant to photoinhibition.