A new species of laevicaudatan branchiopod, Lynceus grandirostris, is described. This new species is so far known only from a single locality near Yellowknife, Canada, and is morphologically distinguished from all other known Lynceus species by the elongated form of the female rostrum, which would appear to preclude copulation. Three collections made over several years have yielded 55 females, but only three males. Males of this species are thus far extremely rare. The large size of the female rostrum coupled with the relative rarity of males suggests that this species may be parthenogenic. A key to the female Laevicaudata of North America is presented