The objectives of this study were to determine the effectiveness ofchemical treatments of the Ocqueoc River, Michigan, in reducing the number of recently metamorphosed sea lampreys Petromyzon marinus migrating to Lake Huron and to estimate total numbers of migrants produced before and after treatment. Sea lampreys were captured during their downstream migration in a single fyke net fished in the same location from September 1963 through August 1975. The catch, which averaged 3,474 sea lampreys (range, 3,248-3,913) during four migration periods (September-June) before treatment in 1968, declined to 4 during the 1974-1975 migration period. Markrecapture studies were conducted to determine the capture efficiency of the net for recently metamorphosed sea lampreys and to estimate the total downstream migration for each migration period. Estimated downstream migrations before treatment averaged 62,036 sea lampreys (range, 58,000-69,875) for four migration periods and declined to 71 during the 1974-1975 migration period. Catches were usually greater in fall than in spring. The fall peak in migratory activity was in November or December, and the spring peak was in April; both peaks occurred while water levels were high and water temperatures were near 5A?C.