Maximum fixation rates of 120 and 60 pmol cm−2 s −1were obtained when exogenous carbon was supplied as 1CO2 and H14CO3− respectively. These values are considerably higher than those previously reported for this species. A kinetic analysis was performed on this data. Substrate saturation in the concentration range 1.0–1.5 mM was observed for both CO2 and HCO3− In the presence of exogenous CO2, a linear relationship was observed between light intensity and fixation while the HCO3− relationship was slightly sigmoidal. Fixation saturated at intensities of 15–20 W m−2 and 13–15 W m−2 for exogenous 14CO2 and H14CO3−respectively. The presence, in this species, of an extremely active HCO3− transport system, situated in the plasmalemma, demonstrates that when alkaline solutions are employed the involvement of this ion cannot be ignored during electrical studies on this membrane. The maximum H14CO3− influxes obtained during this study are the largest ionic fluxes measured for any Characean species. It was demonstrated that CO2 for fixation can be supplied simultaneously by gaseous diffusion and HCO3− transport (cf. Raven, 1968). Inhibition of H14CO3− influx was observed in the presence of Tris, Tricine, and borate buffers, and CO32 − also appeared to act as a strong inhibitor. The possible mechanism(s) by which this inhibition occurs is discussed.