show Abstracthide AbstractThe salmon louse (Lepeophtheirus salmonis), an ectoparasitic copepod on salmonids, has become a major threat for the fish farming industry. In search for new drugs and vaccines RNA sequencing is increasingly in use to find genes and pathways differently regulated due to treatment. However, the underlying gene expression changes going along with the development of lice have to be taken into account. The life cycle of L. salmonis consists of eight stages divided by moults. The development rate of lice on the host is not uniform and individual and sex related differences are found leading to lice of unlike developmental status at same time point after infection. In this study we analyse lice from a time series by RNA sequencing using a method of separating lice of different instar age independent of sampling time point. In total 66 samples comprising lice of 4 stages divided into three different age groups within the stage were analysed in triplicates. The overall gene expression shows that the method for dividing lice was successful. Many genes are expressed cyclic during moulting cycles and overall gene expression differs more between lice of different instar age than between lice of different stage but same instar age.