A program of tagging was carried out in the waters of northern Lake Huron during the fall and winter of 1951–52 in order to supplement the small amount of information available on the movement of sea lampreys during their parasitic phase. A total of 219 parasitic-phase sea lampreys were tagged and released at three localities. Of this number 38 or 17.2 percent were recovered. One tag was recovered near North Manitou Island, Lake Michigan. The remaining 37 were taken in Lake Huron or in streams tributary to that lake. The dispersal of tagged lampreys throughout Lake Huron was wide. Five marked individuals were taken in the southern part of the lake over 150 miles from the point of tagging; 4 of these 5 were captured in Canadian waters. The marked lampreys exhibited no distinct pattern of migration other than a tendency toward a general southeasterly movement in Lake Huron.