In this study we described the cytosolic HSP90 of Bonamia ostreae, an intracellular parasite of Ostrea edulis haemocytes. The complete open reading frame was assembled by Rapid Amplification cDNA Ends reactions on cDNA of B. ostreae infected haemocytes. HSP90 amplification was corroborated in infected oysters and B. ostreae purified cells. The functionality of the HSP90, studied by inhibitory assays with radicicol, suggests that this protein may play a role in haemocyte invasion. Our results inform the molecular basis that governs B. ostreae-O. edulis interactions.