At least one formerly large population of American brook lamprey (Lampetra appendix) has been extirpated in Minnesota, but little is known about the species in the state. We examined densities and age structures of American brook lamprey larvae in several streams in southeastern Minnesota. Mean densities of lamprey larvae in the best habitats available in 13 streams varied from 0.33–5.78 larvae/m². Seven of nine streams examined had missing age classes of larvae, with five streams missing two or more classes. Habitat use and length–weight relationships of larvae were similar to those reported previously for this and other species of lamprey. Many L. appendix populations in Minnesota are healthy, but others are susceptible to extirpation because of low densities and multiple missing age classes. The current status of many of these populations, and their susceptibility to further disturbance, need to be more thoroughly examined to better assess the need for legal protection for the species in Minnesota.