Euglena gracilishas attracted special interest due to its ability to excessively accumulate paramylon. Paramylonis a linear β-1,3-glucan polysaccharideand is the principal energystorage polymer in E. gracilis. The polysaccharide features high bioactive functionality to the immune system. This study explored a new method to enhancethe production ofparamylon byE. gracilisby co-cultivation with the indole-3-acetic acid-producing bacterium, Vibrio natriegens. The enhanced production was achieved indirectlyby phytohormone-producingbacteria, instead of by the direct application of the hormone. The knowledge obtained in this study furthers the understanding ofthe effect of V. natriegenson the growth and physiologyof E. gracilis production.