Some studies have described the use of phytohormones in microalgal culture for the production of biodiesel or selected fatty acids. However, no study has determined the amount of phytohormones that maximizes lipid yield. We determined the optimal concentration of auxins and gibberellins (which is between 40 and 60 μM) in two strains (Scenedemus abundans and Chlorella ellipsoidea) suitable for biodiesel production. More than 3‐fold increment was reached with S. abundans and near 7‐fold increment with C. ellipsoidea. Furthermore, this work suggests that the improved growth of the microalgae in the presence of the phytohormones was due to a reduction in the level of reactive oxygen species (ROS) in the cells. An economic analysis showed that, due to its low cost, auxin offers a positive cost‐benefit balance and therefore could be used at large scale. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1203–1211, 2016