Photosynthesis of Myriophyllum spicatum was measured (14C technique) in the field in waters from eight Italian lakes having different dissolved inorganic carbon (DIC) concentrations and in two mixtures of water from two of the lakes. In the experiments, in which there were some differences in pH and light, photosynthetic response to DIC fit a Michaelis‐Menten relationship, demonstrating the importance of DIC as a factor affecting productivity of these aquatic plants.