Of 2614 adult rainbow trout sampled from the Nottawasaga River of Georgian Bay during the period of 1961–67 inclusive, approximately 7.1% had one or more lamprey marks. The trout ranged in fork length from 10 to 33 inches (25 to 84 cm). However, lamprey marks were present only on fish over 16 inches (40 cm) in length. Multiple scarring was found on 30% of the trout that were marked. The incidence of marked fish reached a maximum of 17.2% in the spring of 1962, then declined to 1.3% in the fall of the same year. This decline was coincident with a reduction in the lamprey population after experimental lampricide treatment of the Nottawasaga River and adjacent lamprey spawning streams during 1960 and 1961. The incidence of marked fish remained at a relatively low level for a period of 4 years, then increased substantially, after the presumed recruitment of metamorphosed lampreys during 1966 and 1967.