28• Behavioural fever is a synergic immune response to infection in ectotherms. 29• Zebrafish larvae (Danio rerio) select their preferred temperature within a 30 vertical gradient tank. 31• The onset of the behavioural fever response was established at 18-20 dpf. 32• Under an immersion challenge with double-stranded RNA (dsRNA) zebrafish 33 larvae display a behavioural fever response coupled to increased antiviral 34 mRNA transcript abundance. 35 Abstract 36Behavioural fever has been reported in different species of mobile ectotherms 37 including the zebrafish, Danio rerio, in response to exogenous pyrogens. In this study 38 we report, to our knowledge for the first time, upon the ontogenic onset of behavioural Keywords 59Zebrafish larvae, thermo-preference, behavioural fever, antiviral response, dsRNA 60 challenge, larval development, temperature choice.