Leaf photosynthesis (A) is limited by mesophyll conductance (g(m)), which is influenced by both leaf structure and the environment. Previous studies have indicated that the upper bound for g(m) declines as leaf dry mass per area (LMA, an indicator of leaf structure) increases, extrapolating to zero at a LMA of about 240 g m(-2). No data exist on g(m) and its response to the environment for species with LMA values higher than 220 g m(-2). In this study, laboratory measurements of leaf gas exchange and in vivo chlorophyll a fluorescence were used concurrently to derive estimates of g(m) in seven species of the Australian sclerophyllous genus Banksia covering a wide range of LMA (130-480 g m(-2)). Irradiance and CO(2) were varied during those measurements to gauge the extent of environmental effects on g(m). A significant decrease of g(m) with increasing LMA was found. g(m) declined by 35-60% in response to increasing atmospheric CO(2) concentrations at high irradiance, with a more variable response (0-60%) observed at low irradiance, where g(m) was, on average, 22% lower than at high irradiance at ambient CO(2) concentrations. Despite considerable variation in A and LMA between the Banksia species, the CO(2) concentrations in the intercellular air spaces (C(i), 262+/-5 micromol mol(-1)) and in the chloroplasts (C(c), 127+/-4 micromol mol(-1)) were remarkably stable.