A model is described for the response of net photosynthesis to inorganic carbon concentration in aquatic plants. This model is a modification of the model for terrestrial plants by Tenhunen et al. (1976) and Tenhunen et al. (1977). An attempt has been made to incorporate into the model HCO3- fixation as well as the increase of internal O2 concentration during photosynthesis and its effect on photorespiration. Factors controlling fixation of inorganic carbon are described in terms of supply of fixable carbon, fixation mechanics, and diffusion resistance to absorption of inorganic carbon. Inorganic carbon supply is the available carbon pool at a distance from the plant where the concentration is unchanged by activities of the plant. Carbon fixation is assumed to follow Michaelis-Menten type kinetics with O2 acting as a competitive inhibitor. Diffusion of free CO2 to the site of fixation is described by Fick’s Law. Transport of HCO3- is assumed to be controlled by a transport system which follows first-order Michaelis-Menten kinetics. Photosysnthetic rate data of Raven (1968) on Hydrodictyon africanum and Lucas (1975) on Chara corallina are successfully simulated (r between .87 and .99, significant at P=.01).