The salmon louse is one of the main problems for the salmon farming industry both in Norway and in other countries. Pesticides and different management measures currently control the salmon louse. However, only a few types of drugs are available for salmo n louse combat and there are several reports for resistant development/treatment failures for the compounds in use (i.e. emamectin benzoate and pyretroids). This raise the need for new control measures and in this respect vaccination is an attractive stra tegy if protective antigens can be identified. Recent studies carried out at IMR, shows that significant reduction in lice number can be achieved through vaccination (see below). This study demonstrates that it is possible to control lice using a vaccinat ion strategy. The antigens used in the first test vaccine are part of a biological process likely controlled by ecdysteroids, either directly or indirectly. Hence, characterisation and identification of genes induced by ecdysteroids seems to be of high re levance as candidate genes in a future L. salmonis vaccine. In the present project proposal we aim to identify and test several new lice vaccine antigens and to establish a new tools for screening of candidate antigens. This includes an extension of our 7 K L. salmonis microarray and to use RNAi as an antigen evaluation tool. The new microarray will be extended by ~5,000 new cDNA probes and used to identify genes induced by ecdysteroids (as measured by the transcription of the ecdysteroid receptor (EcR) (s ee below). RNAi will be established and used as a first line-screening tool. As a final step in antigen evaluation antigens selected based on RNAi results (either directly purified or recombinant) will be tested by immunisation of Atlantic salmon and subs equent challenge with L. salmonis. This includes the EcR, which we have identified in our EST database.