Marking experiments on three streams in the Upper Peninsula of Michigan yielded quantitative estimates of populations of larval and transforming lampreys. The estimates not only gave an idea as to the numbers of ammocetes in the streams, but also confirmed the judgments of abundance based on earlier surveys with electric-shocking equipment and provided valuable information on the movement of larvae during chemical treatment. The estimated numbers of ammocetes in the three streams were 4,300 in Furnace Creek, 30,600 in Snyder Creek, and 336,700 in the Ogontz River.