The distribution of California red fir (Abies magnifica) and noble fir (Abies procera) in Oregon has long been debated. The objective of this study was to determine if selective parasitism of these true firs by subspecies of hemlock dwarf mistletoe (Arceuthobium tsugense) could be used to provide additional data supporting one of the geographic distributions proposed for these conifers. A total of ten mixed-conifer stands in California and Oregon infested with mountain hemlock dwarf mistletoe (Arceuthobium tsugense subsp. mertensianae) and 11 stands in Oregon infested with Pacific silver fir dwarf mistletoe (Arceuthobium tsugense subsp. amabilae) were sampled to compare host susceptibility to these parasitic plants. Populations of true firs morphologically resembling noble fir south of latitude 44° N in Oregon were immune to infection by both dwarf mistletoes. Populations of noble fir north of latitude 44° N in Oregon were principal hosts of Pacific silver fir dwarf mistletoe. The complete immunity of California red fir and trees morphologically resembling noble fir to Pacific silver fir dwarf mistletoe south of approximately latitude 44° N in Oregon supports the classification of these true fir populations as California red fir, and not noble fir. The severe parasitism of trees resembling noble fir by Pacific silver fir dwarf mistletoe north of latitude 44° N in central Oregon supports the classification of these populations as noble fir. These results provide the best example of using dwarf mistletoe-host specificity as evidence to distinguish between different interpretations of the geographic distribution of their coniferous hosts.