the close phylogenetic relationship between ascidians (Tunicata) and vertebrates makes them a powerful model for studying the innate immune system. To better understand the nature and dynamics of immune responses and the mechanisms through which bacterial infections are detected and translated into inflammation in Ciona robusta, we applied an approach combining in vivo lipopolysaccharide (LPS) stimulation, immune-labelling and functional enzymatic analyses. The immunohistochemistry showed that toll-like receptor 4 (TLR4) and nuclear factor kappa B (NFκB) were expressed during the inflammatory pharynx response 4 hours post-LPS, with formation of immune-stained haemocyte nodules in the vessel lumen. Pharynx vessel endothelium also took part in the inflammatory response. Observations of histological sections from naive and MS-inoculated ascidians confirmed immuno-positive tissue response. Enzyme immune parameters – which included the activity of phenoloxidase, glutathione peroxidase, lysozyme, alkaline phosphatase and esterase – showed up-modulation 4 hours after LPS injection, confirming their involvement during ascidian inflammatory response. These findings provide new insights into the mechanisms involved in LPS-induced C. robusta response and suggest that a broad innate immune mechanism is involved in the regulation of tunicate inflammatory responses.