Infestation patterns of Caligus elongatus on farmed Atlantic salmon and rainbow trout were investigated at several sites along the west coast of Ireland. Parasite abundances were examined in relation to host species, farm location and season. Differences were found in the relative prevalence of infestation between salmon and rainbow trout. Caligus elongatus generally contributed more as a proportion of the total lice burden on rainbow trout than on Atlantic salmon. Evidence of possible parasite transmission from wild fish stocks was found at a number of sites where marked seasonal changes in parasite abundance were observed. A wide size distribution of adult female Caligus elongatus was found at a number of sites.