Poa cookii dominates the tussock grasslands at islands of the Kerguelen Province of the sub-Antarctic. In common with other sub-Antarctic Poa species it is highly productive. Infra-red gas analysis studies at Marion Island (46° S, 37° E) showed that P. cookii is a C3 photosynthetic type plant, possessing a high CO2 compensation concentration (c. 100 p.p.m.) and apparently high rates of photorespiration. However, photosynthetic light saturation was not attained at a photosynthetic photon flux density (PPFD) of 2000μmol m−2s−1, possibly due to leaf anatomical features causing shading of the chlorophyllous mesophyll. Compensating irradiance values were 5 to 11 μmol m−2s−1. Net photosynthesis occurred below 0 °C and increased with temperature up to 10 to 12 °C, remaining constant up to 19 to 23 °C, depending in PPFD. Dark respiration increased exponentially with temperature and at 32 °C the temperature optimum had not been reached. Photorespiration also increased with temperature but rates were lower than those of dark respiration. Increasing the wind velocity over a leaf immediately decreased the net assimilation rate, which then slowly recovered to almost the original level. It appears that the grass is well-adapted to sub-Antarctic conditions of low light, low (seldom freezing) temperatures and high wind.