Abstract In the original description of the Alaskan Brook Lamprey Lethenteron alaskense Vladykov & Kott 1978, only the adults of the species were described because the low number of ammocoetes of this species available to the authors (i.e., 4), as well as that of other species of Lethenteron, was deemed inadequate by them to give a proper description of this life history stage. Through fieldwork conducted in the Northwest Territories (N.W.T.), Canada in June-July 2012, augmented by previously catalogued material from the Canadian Museum of Nature, we have now assembled sufficient ammocoete samples to remedy the situation. The purpose of this study is, therefore, to describe the larval stage of the Alaskan Brook Lamprey, L. alaskense and compare it with ammocoetes of the Arctic Lamprey, L. camtschaticum (Tilesius 1811) and the American Brook Lamprey, L. appendix (DeKay 1842); its only congeners in North America (Page et al. 2013). This is important because the Arctic Lamprey is parasitic in the adult stage, wh