a b s t r a c tThe long term effects of moderate elevation ORP (oxidation–reduction potential) around 300–320 mV onthe growth, hematological parameters and the ability of European seabass (Dicentrarchus labrax) to reactagainst bacterial infection was studied in recirculating aquaculture systems (RASs). Two RASs, one with amoderate ozonation (RAS-O3) and a control (RAS-C) were used in this experiment. After 60 days, seabassreared in the RAS-O3were more able to react against a Vibrio anguillarum infection. It was in spite ofthe fact that seabass in the RAS-O3showed decreased feed intake, feed conversion rate, growth rate andmodified hematological parameters compared with the fish in RAS-C. It is obvious that an ORP level of300–320 mV is too high for seabass to adapt in terms of the growth performance and the hematologicalparameters. However the increased ORP resulted in a better ability of the fish to react against bacterialinfection. Our results strongly suggest that ORP for seabass in RAS should be elevated but not exceeding300 mV and a slightly increased and well controlled ORP level (above 240–270 mV) has a positive effecton the disease resistance of fish. For the future, molecular methods could be utilized to identify whichfunctional groups of microbe are contributing to the ORP effect and investigate how ORP influenced fishphysiology in RASs.