Phytoplasmas are plant pathogenic bacteria transmitted by hemipteran insects. The leafhopper Euscelidius variegatus is a natural vector of chrysanthemum yellows phytoplasma (CYp) and a laboratory vector of Flavescence dorée phytoplasma (FDp). The two phytoplasmas induce different effects on this species: CYp slightly improves, while FDp negatively affects insect fitness. To investigate the molecular bases of these different responses, RNA-seq analysis of E. variegatus infected with either CYp or FDp was performed. The sequencing provided the first de novo transcriptome assembly for a phytoplasma vector, and a starting point for further analyses on differentially regulated genes, mainly related to immune system and energy metabolism. Insect phenoloxidase activity, immunocompetence, and body pigmentation were measured to investigate the immune response, while respiration and movement rates were quantified to confirm the effects on energy metabolism. The activation of insect immune response upon FDp infection, which is not naturally transmitted by E. variegatus, confirmed that this bacterium is mostly perceived as a potential pathogen. Conversely, the acquisition of CYp, which is naturally transmitted by E. variegatus, seems to increase the insect fitness by inducing a prompt response to stress. This long-term relationship is likely to improve survival and dispersal of the infected insect, thus enhancing the opportunity of phytoplasma transmission.< Réduire