The protozoan fish parasite Ichthyophthirius multifiliis, causes “white spot” epidemics in a wide range of freshwater fish species worldwide. It infects fish epidermal surfaces such as fins, gills and skin and is responsible for high morbidity and mortality. I. multifiliis inhabits the outermost transparent epidermal layer of fish, providing a unique opportunity to visualize protective immune mechanisms involved in the host-parasite interactions. The zebrafish has in this study been used as a model organism for freshwater fish to investigate the acute immunological reactions towards I. multifiliis and the immune evasive behaviour of the parasite . Larvae from a double transgenic reporter line with green-fluorescent neutrophils and red-fluorescent macrophages were infected with the parasite and the host-parasite interactions were studied using in vivo real-time imaging (Fig. 1) and immune-relevant gene expression. Different host-parasite interactions were observed and are discussed. I t was documented how neutrophils and macrophages were in some instances able to kill the parasite. The parasites created an interstitial space, in which they moved around vigorously while continuously rotating. Based on our results, we hypothesize that one immune evasive strategy of the parasite is to continuously rotate to dodge/escape attacks initiated by fish innate immune cells . Gene expression revealed that a mild acute immune response in zebrafish was induced systemically, supporting the importance of an immediate response to fight this parasite. In this study, new knowledge on host-parasite interactions was obtained and the zebrafish once again proved to be an excellent organism to investigate in vivo reactions.