The occurrence of the C4 photosynthetic system, based upon an examination of leaf anatomy, of nearly all species of the vegetation of Aldabra Atoll, western Indian Ocean, is reported. About 19% of the flora has the C4 system. It only occurs in herbaceous plants, which grow either in grasslands or as an understorey to tall shrubs. Both C3 and C4 species compete side by side in the mixed-species communities.