Large numbers of black flies, mostly Simulium venustum Say, were labelled as larvae with 32P and released in rapids in the Chalk River near the village of Chalk River, Ont., to complete development and disperse as adults. Flies were trapped over a 900 square mile area for 3 months after the release, and radioactive flies from the traps were identified at CRNL by means of an autoradiographic technique. The distance travelled by the flies could be described by a normal distribution curve, with the average migration between 5.8 to 8.2 miles. A few individuals were trapped at a distance of 22 miles 2 days after emergence, and some crossed the Ottawa River which is at least a mile wide at the point nearest the release area. Some traps caught very high numbers of radioactive flies, one at about 6 miles from the release center accounting for more than 1300 tagged flies. The traps giving the highest number of tagged flies (e.g., > 100) were clustered in two distinctive areas, one west and one large area south of the release point. The results indicate that control measures should be applied to black fly streams at least 10 miles from critical areas such as towns.