The effects of growth conditions on paramylon (a β-1,3-glucanreserve carbohydrate) content were examined in the photosyntheticwild-type and a spontaneous non-photosynthetic WZSL mutant of theunicellular flagellate Euglena gracilis. This carbohydrate is known toshow interesting applications in human and veterinary medicine, asimmunostimulant and immunopotentiator. For both strains, the synthesisof reserve depends mainly on the growth conditions, i.e. light or dark. Thehighest amount of paramylon is accumulated by the mutant (90% dryweight) under dark conditions and with glucose as the carbon source. Thesefindings are discussed in terms of feasibility of exploitation of both strainsas an alternative source of β-glucan, and of the importance of thechloroplast compartment in the synthesis of this compound.