Some characteristics of photosynthesis of three synurophyte algae, Synura petersenii, Synura uvella and Tessellaria volvocina were investigated to determine the mechanism of inorganic carbon (Ci) uptake. All three species were found to have no external carbonic anhydrase, no capacity for direct bicarbonate uptake and a low whole-cell affinity for Ci. The internal pH of S. petersenii determined using 14C-benzoic acid and [2-14C]-5,5-dimethyloxazolidine-2,4-dione was pH 7.0–7.5, over an external pH range of 5.0–7.5. Thus, the pH difference between the cell interior of S. petersenii and the external medium was large enough, over the alga’s growth range, to allow the accumulation of Ci by the diffusive uptake of CO2. Monitoring O2 evolution and CO2 uptake by suspensions of S. petersenii at pH 7.0 by mass spectrometry did not indicate a rapid uptake of CO2, and the final CO2 compensation concentration reached was 24 ± 0.7 μM. Furthermore, when the cells were darkened, a brief burst of CO2 occurred before a steady rate of dark respiration was established, suggesting a loss of CO2 by photorespiration. An examination of the kinetics of ribulose-1,5-bisphosphate carboxylase/oxygenase in homogenates of cells of S. petersenii, S. uvella and Mallomonas papillosa showed that values of the Km (CO2) were 28.4, 41.8 and 18.2 μM, respectively. These species lack the characteristics of cells with a CO2-concentrating mechanism because the cell affinity for Ci appears to be determined by the relatively high CO2 affinity of the Rubisco of these algae.