On the basis of annual observations collected over 35 y, we chronicled the trends in abundance of Mule Deer (Odocoileus hemionus) and White-tailed Deer (Odocoileus virginianus) in semi-open montane habitat in the Devona district of Jasper National Park (JNP), Alberta, 1981–2016. During 722 d of observations conducted in winter over this period, we recorded a decline in Mule Deer and the incursion of White-tailed Deer into the park. Of a total of 429 deer sighted, White-tailed Deer increased from an average of 0.08 sightings/d to 0.73/d, whereas the native Mule Deer declined from 0.42 sightings/d to 0.01/d. Over the same time span, sightings of all deer increased from 0.51/d to 0.74/d. Although the ultimate cause of the opposing population trends of the 2 deer species is not certain, we review the proximate causes discussed in relevant literature, and we compare the results of our census to a list of deer killed by vehicle collisions on JNP roads and highways. As a measure of the presence of Gray Wolves (Canis lupus) in the study area, we recorded the largest size of wolf packs sighted each year and found no trend over time. The question of whether wolf predation on the 2 deer species could account for their opposing population trends remains to be investigated.