Metamorphosed sea lampreys (Petromyzon marinus) were collected and marked at two points in their life cycle. Recently metamorphosed juveniles were collected from streams, marked with coded wire tags, and returned to migrate to the Great Lakes. Juveniles already in the lakes and feeding on teleost hosts were obtained from incidental catches by sport or commercial fisheries. Sea lampreys in the Great Lakes spend only 1 feeding year as parasites, and marked animals were recaptured during the spawning runs. For one marked group in each of four parasitic cohorts (feeding years 1991 to 1994) and two marked groups in each of three cohorts (feeding years 1998 to 2000) we recovered from 1.1 to 10.2 percent of marked animals. The number of metamorphosed animals present in autumn before migration to Lake Huron was estimated for five cohorts, with estimates ranging from 639 to 803 thousand. The number of feeding, parasitic animals present in Lake Huron in mid summer was estimated for five cohorts, with estimates ranging from 515,000 to 2,342,000. The larger estimates later in the parasitic year suggested that animals collected and marked from sport or commercial fisheries did not survive at the same rate as unmarked animals. It is recommended that only estimates from recaptures of animals marked in the streams before migration be used until it can be established why survival of juveniles obtained from sport or commercial fisheries might be affected.