A remarkable feature of many parasites is a high degree of host specificity but the mechanisms behind are poorly understood. A major challenge for parasites is to identify and infect a suitable host. Many species show a high degree of host specificity, being able to survive only on one or a few related host species. To facilitate transmission, parasite's behavior and reproduction has been fine tuned to maximize the likelihood of infection of a suitable host. For some species chemical cues that trigger or attract the parasite in question have been identified but how metazoan parasites themselves receive these signals remains unknown. In the present study we show that ionotropic receptors (IRs) in the salmon louse are likely responsible for identification of a specific host. By using RNAi to knock down the expression level of different co-receptors, a significant change of infectivity and settlement of lice larvae was achieved on Atlantic salmon. More remarkably, knock down of the IRs changed the host specificity of the salmon louse and lice larvae settled at a significant rate on host that the wild type lice rejected within minutes. To our knowledge, this has never before been demonstrated for any metazoan parasite. Our results show that the parasites are able to identify the host quickly upon settlement, settle and initiate the parasitic life style if they are on the right host. This novel discovery opens up for utilizing the host recognition system for future parasite control.