Parasitism involves intimate interactions between the two species at many levels, e.g. behavioral changes or modulation of the immune system.</p><p>Little is known about host-parasite interactions concerning transcriptome co-plasticity. The salmon louse (Lepeophtheirus salmonis), an ectoparasitic copepod on salmonids, has become a major threat to the fish farming industry.</p><p>We have conducted a coordinated transcriptomics study of a marine ectoparasite and its vertebrate host (Salmo salar) by simultaneous whole transcriptome shotgun sequencing. We compared gene-expression between parasites fed on different families of salmon genetically selected for either higher (HR) and lower resistance (LR) towards the salmon louse and between parasites feeding on different body parts of the fish.</p><p>Parasite samples are linked to the individual fish they have been collected from by the PIT-tag ID of the host. Both host and parasite transcriptomes were sequenced separately. Salmon samples are deposited in a different BioProject [TBA].