Chemoreception is critical for marine ectoparasites - such as salmon lice (Lepeophtheirus salmonis) - to identify and locate salmonid hosts. The molecular receptors that parasites employ to detect host-specific chemical stimuli from hosts (kairomones) have not been well characterised. In the present study, transcription of the sea louse Ionotropic receptor 25a (IR25a) was blocked to evaluate whether it functions as a chemical-perception related gene for a specific chemical cue from the Atlantic salmon host. Double-strand RNA interference (dsRNA) oligonucleotides were applied to salmon lice by in vitro transcription and then exposing salmon lice nauplii to dsRNA by soaking overnight. Silencing of the IR25a gene was confirmed by qPCR in experimental groups of knock-down copepodids (dsIR25a). Behavioural responses associated with host recognition were evaluated in dsIR25a sea lice after exposure to a peptide produced by the salmon host (Cath-2). The dsIR25a group decreased expression levels of IR25a by >7-fold with respect to the control group. This group was also 26% slower than the control group (control swimming speed was 69 mm/s, while the treated group was 51 mm/s). Since the swimming activity of salmon lice copepodids is associated with the activation of the chemosensory system, these results indicate that the L. salmonis chemosensory perception system was not fully activated due to gene silencing. The results of this study demonstrate the role of ionotropic receptor 25a during host recognition by sea lice.