The reliable production of marine fish larvae is still one of the major bottlenecks in aquaculture due to high mortalities mainly caused by the outbreak of infectious diseases. Therefore, the present study aimed to explore the capacity of poly-ß-hydroxybutyrate (PHB) to improve the larval performance and immunity in European sea bass (Dicentrarchus labrax) yolk-sac larvae in order to evaluate if PHB is a suitable candidate for immunostimulation in sea bass larviculture.