The influence of light intensity on growth of the freshwater microalga Chlorella pyrenoidosa has been studied. A mineral culture medium supplemented with glucose at concentrations of 0.1, 0.5 and 1 g litre(-1) was used. The different growth phases observed have been described. In the exponential mixotrophic phase, the specific growth rate did not depend on light, since the algae were able to use glucose as an energy source, a fact demonstrated by the increase in the specific rate of glucose consumption on reducing the light intensity. When the organic substrate was completely consumed, and after an adaptation period, an autotrophic phase followed. The results show that it is advisable to add glucose only to light-limited cultures. The highest biomass/substrate yields are attained at 0.1 g litre(-1).