In Canada, Lyall's Mariposa Lily, Calochortus Iyalllii, is confined to a single height of land in extreme southcentral British Columbia, where it occupies natural openings in Douglas-fir forest. The ll known colonies in this upland area represent the northern range limit of the species, which is otherwise limited to the eastern slopes of the Cascade Mountains in Washington. Aside from gravity, Calochortus lyallii possesses no obvious mechanism for dispersing its seeds, a factor that may limit its ability to establish new populations at unoccupied sites. Nevertheless, Calochortus lyallii tends to be abundant where it occurs (median estimated patch size = 6500+ individuals), and colonies in British Columbia appear to be robust, both in terms of numbers of individuals and the proportion of new recruits present. Howeve, recent silvicultural activities on Black Mountain, such as the introduction of forest tree seedlings into previously untreed Calochortus lyallii habitat, together with ongoing disturbances associated with livestock grazing (trampling, establishment of weedy exotics), now pose a serious potential threat to the persistence of Calochortus lyallii in Canada. It is suggested that the most effective way of safeguarding critical habitat for this species would be to safeguard the entire grass-forb community of which it is a part. © The Ottawa Field-Naturalists' Club. Abstract reproduced by permission.