The physiological responses of four potato (Solanum tuberosum L.) cultivars to continuous irradiation were `determined in a controlled environment. Under a constant 18C and a constant photoperiod of 470 μmol·s-1·m-2 of photosynthetic photon flux, `Denali' and `Haig' grew well and produced large plant and tuber dry weights when harvested 56 days after transplanting. `Kennebec' and `Superior' were severely stunted, producing only 10% of the plant dry matter produced by `Denali' and `Haig'. The differences in leaf chlorophyll concentration and stomatal conductance were not consistent between these two groups of cultivars. The leaf net CO2 assimilation rates in `Kennebec' and `Superior' were lower, and intercellular CO2 partial pressures were higher than in `Denali' and `Haig'. These results indicate that inhibition of net CO2 assimilation in `Kennebec' and `Superior' was not due to a limiting amount of chlorophyll or to CO2 in the leaf tissues. Concentrations of starch in leaflets of `Kennebec' and `Superior' plants were only 10% of those in `Denali' and `Haig' plants, although soluble sugar concentrations were similar in the four cultivars. Therefore, the lower net CO2 assimilation rates in stunted `Kennebec' and `Superior' plants were not associated with an excess carbohydrate accumulation in the leaves.