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Ozone (O 3 ) dissolved in seawater (ODS) was evaluated, as an egg disinfectant, on the spawn of captive gilthead sea bream, Sparus aurata, brood stock. Four contact times (CT) were tested (0.6, 1.2, 2.4 and 4.8 mg min L À 1 ) where CT was calculated by multiplying the dissolved O 3 concentration (0.3 mg L À1 ) by di¡erent exposure periods (2, 4, 8, 16 min). There was also a disinfected seawater treatment that contained no O 3 or derived compounds (CT 0) and an untreated seawater control. All ODS treatments reduced egg surface bacterial counts to zero, which was signi¢cantly (Po0.05) lower than the CT 0 and the control groups (194 and 1320 plate À1 respectively). Nevertheless, the hatching rate was high in the control and the CT treatments 0, 0.6 and 1.2 (88.7%, 87.3%, 89.5% and 83.7% respectively) while eggs exposed to a CT 2.4 and 4.8 hatched poorly (36.5% and 20.4% respectively), which was likely due, at least in part, to larvae unable to break the egg chorion successfully. Swim-bladder in£ation was signi¢cantly higher in the ODS groups (497%) compared with the control and CT 0 treatments (ca.70%). The results suggest that a 2-min exposure of eggs to 0.3 mg O 3 L À1 of ODS (CT 0.6) would improve current protocols in marine larviculture.