The effects of light intensity and carbon dioxide (CO) concentration on the growth of the highly CO2-tolerant green alga Chlorococcum littorale were studied in batch cultures. Four mathematical representations were compared for the specific growth rate-light response curve: a rectangular hyperbolic function, Steele's exponential function, a Poisson function and a hyperbolic tangent function. The hyperbolic tangent function, which is commonly used for representing the photosynthesis-light relationship, gave the best fit as evaluated by the extended information criterion (EIC). EIC proved to be applicable as a criterion to this kind of nonlinear model selection problem. Carbon dioxide, the sole carbon source for photoautotrophic growth of this alga, inhibited the growth rate at concentrations higher than pCO(2) of 0.02. A substrate inhibition model was successfully used to simulate the relationship between the specific growth rate and CO2 response.