Several species of sea lice, in particular Lepeophtheirus salmonis (Krøyer), affect the welfare and condition of farmed salmon, with an estimated annual cost to the Scottish industry of 15-30 Pounds million. In Atlantic salmon, some stocks show resistance to L salmonis. Such natural resistance could be utilized for stock improvement using molecular genetic technologies. The development of molecular markers linked to resistance genes, allowing the identification of resistant fish, could increase the efficacy of selective breeding programmes. Various approaches to achieve this goal are described. One way to identify genes conferring resistance is to develop screens for salmon genes that are activated upon louse infection. One such screen--differential display--requires no previous knowledge of gene sequences, involves no preconceptions about which gene families are involved and can therefore identify novel genes. Preliminary results of comparative gene expression in sea-louse-challenged and control fish illustrate the application of differential display.