The sea louse (Lepeophtheirus salmonis Krøyer) is a major health problem for both farmed and wild salmonids. This paper investigates louse epidemiology and management in the salmon-farming zone of western Scotland. Based on a review of the marine ecology of wild salmon (Salmo salar L) and sea trout (Salmo trutta L), and catch and farm production statistics, best estimates were made for numbers of wild and farm hosts present in coastal waters in March-June 2000. Applying data for ovigerous female louse infections and fecundity, the sources and risks of larval transmission to wild salmon and sea trout were modelled. Farm salmon in the second spring of production were the primary host group (98% of fish), while numbers of wild salmonids (< 1%) and escaped farm salmon (2%) were relatively insignificant. Farm salmon produced 97% of louse eggs at high levels (eight ovigerous lice per fish), and 78% at low levels (one per fish). Wild salmonids produced < 1% of eggs under both scenarios, but escaped farm salmon produced 3% and 21%, respectively. All hosts potentially cross-infect one another, but farm salmon are more likely to infect wild and farm smolts, and also other farm salmon. Monitoring of lice on sea trout in June 1998-2000 by the Association of West Coast Fisheries Trusts corroborated the model's conclusions. Localised epizootics occurred every year and coincided with the presence of ovigerous lice on local farms. In areas of mixed-year class production on farms, epizootics were evident every spring, but occurred every second spring in areas of single-year class production. In 1998-2000 at least 14-40% of sea trout were infected with potentially lethal infestations of lice. Ovigerous louse levels of < 0.005 per fish were required on farm salmon in the spring of 2000 to produce less eggs than those emitted by wild salmonids. With the industry's continued expansion, and thus increased numbers of farm salmon, a target of zero ovigerous lice will be required on farms to minimise impacts on wild salmonids. Due to the limited long-term efficacy and availability of louse medicines, management strategies are discussed which will improve control, including single-year class production over large areas, alternate S1-S1/2 smolt inputs, and 11-month production cycles.