The utilization of inorganic carbon in filaments of Scytosiphon lomentaria was investigated by pH-drift technique and pH electrode method combined with related inhibitors. The influence of different concentrations of carbon dioxide on the activity of intracellular carbonic anhydrase (CA) was also explored at the same time.The results showed that (1)the pH compensation point of filaments of S. lomentaria was 9.21 and the CO₂ compensation point was 0.74 μmol/L.Acetazolamide (AZ) has no significant influence on the pH compensation and the inorganic carbon uptake amount in the first three hours of the inhibitor experiment. Ethoxyzolamide (EZ) and vanadate (Van) decreased the pH compensation point to the same value 8.82 and the inorganic carbon uptake amount inhibition rates were 55.93% and 56.72%, respectively, in the first three hours of the inhibitor experiment. Under natural conditions, filaments of S. lomentaria use free CO₂ and HCO₃^- as inorganic carbon source, and the utilization of HCO₃^- depends on ATPase and intracellular CA. (2) 0.35%, 0.70% and 1.20% CO₂ can inhibit the activity of intracellular CA of filaments of S. lomentaria and the inhibition rates were 6.29%, 17.44% and 27.15%, respectively. The higher CO₂ concentration, the stronger inhibition effect of the activity of intracellular carbonic anhydrase.