ABSTRACT: Little is known about migratory routes and habitat use of anadromous brown trout Salmo trutta at sea. We therefore conducted a 2 yr study (2011-2012) on sea trout in the River Himleån system (a river, an estuary, and a coastal sea environment) in western Sweden. The main objectives were to investigate migration speed, migration routes, and mortality both for adult brown trout returning to the sea (kelts) and smolts (generally 2 yr old trout migrating to the sea for the first time). Brown trout were captured and tagged with hydro-acoustic transmitters, after which they were released and monitored during their initial migration. Migration was primarily nocturnal in the river and estuary, whereas no tendency for nocturnal migration was observed along the coast. Migration speed tended to decrease as individuals progressed from the river through the estuary and along the coast, and we found no differences in absolute migration speed between kelts and smolts. Smolts and kelts showed similar mortality. Mortality during the initial part of the migration ranged between 5 and 51% and was significantly higher in 2011, for both smolts and kelts. Our study is the first to compare migratory patterns and mortality rates between sea trout kelts and smolts during the transition from freshwater into an open coastal system.