When Euglena gracilis was grown in the heterotrophic condition with glucose and (NH4)(2)SO4 as the carbon and nitrogen source, a high cell yield (4.28-4.48 g l(-1)) was obtained and the culture pH decreased to 1.6-2. The biomass production in the heterotrophic culture was compared to those in the autotrophic and mixotrophic cultures. Autotrophic growth was 4.7-6.3% of the heterotrophic one, whereas about 15-19% higher growth was obtained in the mixotrophic culture. Moreover, good production of chlorophyll (39.4 mg l(-1)) and carotenoids (13.8 mg l(-1)) were attained in the mixotrophic culture, giving the highest fermenter productivity with respect to biomass as well as chlorophyll and carotenoids. Through an energetic analysis in the mixotrophic culture, it was estimated about 25-28% of the total ATP requirement is formed in the photochemical reactions. This resulted in an improved biomass production in the mixotrophic culture of E. gracilis.