A large caprellid amphipod recently discovered at a salmon farm in the Lynne of Lorne near Oban, Scotland, has been identified as Caprella mutica, a species indigenous to north-east Asia. The caprellid population appears to have become established in the last four years at the site with a high abundance of animals occurring year round on the farm nets, mooring ropes and on artificial experimental structures located <10 m from the fish farm. This paper briefly describes C. mutica found in Scotland. Application of Chapman & Carlton's criteria for determining introduced species suggests that C. mutica is non-indigenous to the area.