Bacterial production of CO2 from sucrose substrate increased growth of seven species of algae of CO2-limited laboratory cultures. Decomposition of organic matter in pond water also supplied enough CO2 to support good algal growth in cultures deprived of other sources of CO2. Estimates of CO2 production from decay of dissolved organic matter in six pond waters ranged from 0.32 to 3.53 mg/L per 24 hr. The carbonate-bicarbonate equilibrium system is a major source of CO2 for algal photosynthesis. However, in waters of low or extremely high alkalinity, this system will not support high rates of photosynthesis. Decomposable organic compounds must be considered with nitrogen and phosphorus as factors responsible for accelerated eutrophication and nuisance algal blooms.