A simple model of photosynthetic CO2 assimilation in Chlamydomonas has been developed in order to evaluate whether a CO2-concentrating system could explain the photosynthetic characteristics of this alga (high apparent affinity for CO2, low photorespiration, little O2inhibition of photosynthesis, and low CO2 compensation concentration). Similarly, the model was developed to evaluate whether the proposed defects in the CO2-concentrating system of two Chlamydomonas mutants were consistent with their observed photosynthetic characteristics. The model treats a Chlamydomonas cell as a single compartment with two carbon inputs: passive diffusion of CO2, and active transport of HCO3-. Internal inorganic carbon was considered to have two potential fates: assimilation to fixed carbon via ribulose 1,5-bisphosphate carboxylase-oxygenase or exiting the cell by either passive CO2 diffusion or reversal of HCO3-transport. Published values for kinetic parameters were used where possible. The model accurately reproduced the CO2-response curves of photosynthesis for wild-type Chlamydomonas, the two mutants defective in the CO2-concentrating system, and a double mutant constructed by crossing these two mutants. The model also predicts steady-state internal inorganic-carbon concentrations in reasonable agreement with measured values in all four cases. Carbon dioxide compensation concentrations for wild-type Chlamydomonas were accurately predicted by the model and those predicted for the mutants were in qualitative agreement with measured values. The model also allowed calculation of approximate energy costs of the CO2-concentrating system. These calculations indicate that the system may be no more energy-costly than C4 photosynthesis.