To address the question of whether free CO2 can be a growth-limiting nutrient in oceanic waters 3 large marine diatoms, Stephanopyxis palmeriana (Greville) Grunow, Ditylum brightwellij (T. West) Grunow in Van Heurck 1883, and Coscinodiscus sp., were grown in pH-drift experiments under batch culture conditions. The cultures were maintained under quiescent conditions without added buffer in the growth medium, allowing the pH to rise and the free CO2 concentration to fall as growth proceeded. Growth rates were constant for sustained periods and only decreased as the pH rose to as high as 8.5-8.6 and the free CO2 concentration fell to -4 1.1mol l-' Such a low free CO2 concentration is far below the expected half saturation coefficient for ribulose biphosphate carboxylase-oxygenase (RUBISCO) and suggests that these species were capable of utilizing HC03in some fashion so as not to allow free CO2 to become growth-limiting. Additional experiments were conducted with these and other diatoms to demonstrate that turbulent mixing at high pH levels had no effect on inorganic carbon uptake or growth rate and that HCO, uptake probably was occurring. Turbulent mixing should have enhanced inorganic carbon uptake by lowering diffusion gradients had these species been obligate users of free COz. These species, being the most susceptible to diffusion-controlled uptake of free COz because of their long diffusion paths, represent the worst case scenario for free CO2 limitation. Finally, by developing a chemical-biological model it was possible to show that, in order for the pH of marine waters to rise even several tenths above ambient levels, biomass concentrations must increase to levels that are never found, except poss~bly on occasion in estuarine or coastal waters. Thus ~t seems unlikely that free CO2 limits phytoplankton growth in the oceans.