The potentially toxic cyanobacterium Cylindrospermopsis raciborskii (Wo??oszy??ska) Seenya et Subba Raju, originally described as a tropical-subtropical species, is increasingly found in temperate regions and its range is expanding. Climate change is hypothesised to be a factor in this expansion. We studied the effects of dissolved inorganic carbon (DIC) and pH on growth and photosynthesis of this species. We prepared six treatments in a continuous culture (turbidostat) grown at high light, two at low light, and eight in batch cultures grown under low light, by manipulating pH, HCO3- and CO2 to assess the effect, if any, of these parameters on the growth rate, inorganic carbon acquisition and photosynthetic parameters of C. raciborskii. When the turbidostats were grown at 100 I?mol photons (PAR) m-2 s-1, HCO3- concentration and pH had a positive effect on growth rate; the specific growth rate in 6 mM HCO3-, for example, was twice what it was in 0.6 mM HCO3- (0.84 A? 0.10 and 0.44 A? 0.04 d-1 respectively). Growth was lower in turbidostats grown at 20 I?mol photons (PAR) m-2 s-1. Isotope disequilibrium experiments showed that the contribution of HCO3- to DIC acquisition is proportionately greater at the higher light. The maximum relative electron transport rate (rETRmax) was significantly higher at the higher light, while the slope of the linear portion of the rETRmax versus irradiance curve (I?) was unchanged. In low light batch cultures, increasing HCO3- concentration and pH had a negative effect on growth, while CO2 concentration had a small, positive effect. This species of cyanobacterium has an efficient CCM and under ideal growing conditions gets most of its carbon from HCO3-. It may, therefore, be at a competitive disadvantage in a high CO2 world.