The effect of various concentrations of NaCl in the culture solution (up to 400 mM) on growth (fresh weight, dry weight, chlorophyll content) and net CO₂ gas exchange was studied in two halophytes, Mesembryanthemum nodiflorum L. and Suaeda maritima (L.) Dum. Mesembryanthemum nodiflorum reaches optimal growth at concentrations of 100 to 200 mM NaCl in the culture medium, whereas growth of Suaeda maritima is stimulated up to concentrations of 400 mM NaCl. Independent of this stimulated plant growth, net CO₂ uptake in the light appears to be decreased with increased salinity when net CO₂ assimilation is calculated on a fresh weight or dry weight basis. However, expressed per mg chlorophyll, net CO₂ uptake is clearly correlated with plant productivity at different levels of salinity.