The effects of current velocity on the photosynthesis of the submerged angiosperm Callitriche stagnalis Scop. were investigated by CO2-exchange in a closed water-flow system. Apparent photosynthesis was stimulated by increasing velocities and a maximum rate was reached at 8–12 mm s−1. Increasing the velocity to 20 and 40 mm s−1 reduced the photosynthetic rates by 5–30% and 13–29%, respectively. It is suggested that the inhibition of photosynthesis at high velocities is caused by agitation of the incubated plant material. The ecological significance of this result is discussed briefly.