Leaves ofAlternanthera philoxeroides, alligator weed, developed at a photosynthetic photon flux density (PPFD, light energy at wavelengths of 400 to 700 nm) of 790 μmol sec−1m−2(High Light) had less surface area, were thicker, had a higher maximum Pn(net rate of CO2uptake), and required a higher PPFD for saturation of Pn, than leaves developed at 160 μmol sec−1m−2(Low Light). Mesophyll thickness at Low Light was within 19% of maximum 2 days after emergence but at High Light, thickness increased 79% between 2 and 16 days after leaf emergence. The ratio of mesophyll surface area to leaf surface area decreased during development in both light treatments; the ratio, however, was over 70% greater in fully expanded High Light leaves than in Low Light leaves. Maximum Pnexpressed on a leaf surface area basis was 158% greater in High Light leaves than in Low Light leaves, but Pnwas only 58% greater when expressed on a mesophyll surface area basis. It was estimated that fully expanded High Light leaves fixed 72% more CO2per leaf (Pnexpressed per unit surface area times the total surface area per leaf) than fully expanded Low Light leaves when Pnwas measured at the PPFD leaves expanded under. Both High and Low Light leaves would fix about the same amount of CO2per leaf when Pnwas measured at 160 μmol sec−1m−2because the larger surface area of the Low Light leaves offset small differences in Pn.