Spread and buildup of dwarf mistletoe, Arceuthobiun abietinum, was studied on inoculated white fir, Abies concolor, and red fir, A. magnifica, in northern Califomia for 23 to 28 years. At the end of these studies (1986), and in the absence of overstory infection, 13 of 23 trees had dwarf mistletoe populations that were the same or smaller than the original populations resulting from inoculation. Mortality of infections was the main factor limiting population increases. Live crown ratio of all trees averaged over 0.8. The average ratio of tree height growth to vertical spread rate of dwarf mistletoe was 11.5 to 1 in white fir and 7 to 1 in red fir in the Sierra Nevada. In the southem Cascades, the average ratio was 1.7 to 1 in red fir. About one fourth of the trees became infected in the bole. Of 14 additional trees infected by lateral spread of the parasite, 13 were within 6 m of the source of infection. Evidence continues to indicate that losses from dwarf mistletoes will be small in well-managed young fir stands free from infected overstory trees and properly spaced to promote good growth.