Photosynthetic uptake of H14CO3−by phytoplankton in a small freshwater pond was measured on six different dates. Nitrate and phosphate concentrations did not limit the photosynthetic rates of these phytoplankton. Light intensity and CO2concentrations did affect photosynthesis, with dissolved CO2definitely limiting photosynthesis on two of the dates and possibly limiting it on a third date. CO2-limitation of photosynthesis occurred even in the presence of high concentrations of HCO3−(2–3 mM) on days when there was a large standing crop of actively photosynthesizing phytoplankton.