Leaf anatomy, rates of photosynthesis, leaf N, chlorophyll, RuBP carboxylase and nitrate reductase were studied on the indigenous Agrostis magellanica Lam. and the invasive alien Agrostis stolonifera L. on Marion Island (46° 54′ S, 37° 45′ E). Leaves of A. magellanica were more deeply ridged, thicker and more sclerophyllus than those of A. stolonifera. Mesophyll cells of A. magellanica were larger but the number of cells per unit leaf area and the total area of chloroplast per unit leaf area were the same for the two species. Maximum CO2 assimilation rates for the two species did not differ (mean maxima of 9.5 and 9 μmol CO2 m−2s−1for A. magellanica and A. stolonifera respectively). At low photon flux densities, A. stolonifera showed a greater response of CO2 assimilation to photon flux density. A. magellanica exhibited temperature-dependent photoinhibition. Leaf N, chlorophyll and RuBP carboxylase on a fresh or dry weight basis were higher in A. stolonifera but on a leaf area basis there was little difference between the species. The competitive ability of A. stolonifera on Marion Island may be related to its response to low photon flux densities or to its carbon allocation patterns (less sclerophyllous tissue means that a greater leaf area may be produced per unit carbon fixed). The lack of support tissue may limit it to sites partly sheltered from frequent gale force winds.